GENEVA U.N. human rights investigators have gathered testimony from casualties of Syria's civil war and medical staff indicating that rebel forces have used the nerve agent sarin, one of the lead investigators said on Sunday. 
 
 The United Nations independent commission of inquiry on Syria has not yet seen evidence of government forces having used chemical weapons, which are banned under international law, said commission member Carla Del Ponte. 
 
 "Our investigators have been in neighboring countries interviewing victims, doctors and field hospitals and, according to their report of last week which I have seen, there are strong, concrete suspicions but not yet incontrovertible proof of the use of sarin gas, from the way the victims were treated," Del Ponte said in an interview with Swiss-Italian television. 
 
 "This was use on the part of the opposition, the rebels, not by the government authorities," she added, speaking in Italian. 
 
 Del Ponte, a former Swiss attorney-general who also served as prosecutor of the International Criminal Tribunal for the former Yugoslavia, gave no details as to when or where sarin may have been used. 
 
 The Geneva-based inquiry into war crimes and other human rights violations is separate from an investigation of the alleged use of chemical weapons in Syria instigated by U.N. Secretary-General Ban Ki-moon, which has since stalled. 
 
 President Bashar al-Assad's government and the rebels accuse each another of carrying out three chemical weapon attacks, one near Aleppo and another near Damascus, both in March, and another in Homs in December. 
 
 The civil war began with anti-government protests in March 2011. The conflict has now claimed an estimated 70,000 lives and forced 1.2 million Syrian refugees to flee. 
 
 The United States has said it has "varying degrees of confidence" that sarin has been used by Syria's government on its people. 
 
 President Barack Obama last year declared that the use or deployment of chemical weapons by Assad would cross a "red line". 
 
 (Reporting by Stephanie Nebehay; Editing by Tom Pfeiffer) ||||| BEIRUT (Reuters) - A Syrian rescue service operating in rebel-held territory said on Tuesday a helicopter dropped containers of toxic gas overnight on a town close to where a Russian military helicopter had been shot down hours earlier. 
 
 The opposition Syrian National Coalition (SNC) accused President Bashar al-Assad of being behind the attack. Assad has denied previous accusations of using chemical weapons. 
 
 A spokesman for the Syria Civil Defence said 33 people, mostly women and children, were affected by the gas, which they suspect was chlorine, in Saraqeb, in rebel-held Idlib province. 
 
 The group, which describes itself as a neutral band of search and rescue volunteers, posted a video on YouTube apparently showing a number of men struggling to breathe and being given oxygen masks by people in civil defense uniforms. 
 
 “Medium-sized barrels fell containing toxic gases. The Syrian Civil Defence was not able to determine the type of the gas,” said the spokesman. 
 
 The Syrian government and its Russian allies were not immediately available for comment. 
 
 Related Coverage U.S. says looking into reports of toxic gas dropped on Syrian town 
 
 Later, state news agency SANA said rebels had fired rockets armed with toxic gas on the government-held old quarter of Aleppo city, killing five people and causing eight breathing difficulties. It gave no further details. Rebels have denied previous accusations of using chemical weapons. 
 
 The SNC said of the reported use of poison gas in Saraqeb: “After shelling, besieging and killing civilians and perpetrating war crimes on them, the Assad regime has resorted once again, and in breach of UN resolutions 2118 and 2235, to using chemical substances and toxic gases. 
 
 “The daily reality confirms that all the international agreements and previous security council decisions, be they about chemical weapons or otherwise, are meaningless for the Assad regime.” 
 
 The Civil Defence spokesman said it was the second time Saraqeb had been hit by toxic gas. The group was aware of around nine suspected chlorine gas incidents across Idlib province since the conflict began, he said. 
 
 The U.S. State Department said it was looking into the reported use of chemical weapons in Saraqeb. 
 
 “I’m not in a position to confirm the veracity of (the reports),” said spokesman John Kirby. “Certainly, if it’s true, it would be extremely serious.” 
 
 A still image taken on August 2, 2016 from a video said to be taken on August 1, 2016 posted on social media shows a man sitting breathing through a mask in what is said to be in Saraqeb, Idlib province, Syria. Social Media/Handout via Reuters Tv 
 
 Monitors at the Britain-based Syrian Observatory for Human Rights, which tracks violence on all sides in the civil war, said barrel bombs fell on Saraqeb late on Monday, wounding a large number of citizens. 
 
 Russia’s defense ministry said a Russian helicopter was shot down near Saraqeb during the day on Monday, killing all five people on board, in the biggest officially acknowledged loss of life for Russian forces since they started operations in Syria. 
 
 DENIALS 
 
 The helicopter came down roughly mid-way between Aleppo and Russia’s main air base at Hmeimim in the western province of Latakia, near the Mediterranean coast. 
 
 Russian air power began supporting Syrian President Bashar al Assad late last year, an intervention which tipped the balance of the war in Assad’s favor, eroding gains the rebels had made that year. 
 
 No group has claimed responsibility for downing the Mi-8 military transport helicopter. 
 
 Government and opposition forces have both denied using chemical weapons during the five-year-old civil war. Western powers say the government has been responsible for chlorine and other chemical attacks. The government and Russia have accused rebels of using poison gas. 
 
 Slideshow (4 Images) 
 
 U.N. investigators established that sarin gas was used in Eastern Ghouta in 2013. The United States accused Damascus of that attack, which it estimates killed 1,429 people, including at least 426 children. Damascus denied responsibility, and blamed rebels. 
 
 Later that year the United Nations and the Syrian government agreed to destroy the state’s declared stockpile of chemical weapons, a process completed in January 2016. 
 
 The Organisation for the Prohibition of Chemical Weapons confirmed in late 2015 that sulfur mustard, commonly known as mustard gas, had been used for the first time in the conflict, without saying which party in the many sided conflict it thought had used it. ||||| Media playback is unsupported on your device Media caption A child describes his mother "falling to the floor", as James Robbins reports 
 
 France's foreign minister has said a "reaction with force" could be needed if Syria is proved to have used chemical weapons against civilians. 
 
 Laurent Fabius's comments come a day after Syrian activists said hundreds of people died in such attacks in the Ghouta area of the capital, Damascus. 
 
 The UN has asked Syria to allow UN weapons inspectors already in the country to be allowed to investigate. 
 
 But there is no sign as yet that Damascus will allow this. 
 
 The UN team arrived in the city on Sunday and are staying about 15km (10 miles) from the site of the recent attacks. 
 
 Analysis The chances of the UN chemical weapons inspectors in Syria accessing the true site of Wednesday's alleged chemical attack in time to make a clear judgement on responsibility are slim. It took months to negotiate permission for them to visit other sites around the country. The Syrian government, backed by Russia, is resisting calls to give them access to the eastern Damascus suburb of Ghouta where this apparent atrocity took place. Part of the reason is the area is contested between government forces and rebels and is therefore unsafe. If an agent such as sarin has been used, the UN team would need to get to the site within days before traces become so faint as to be inconclusive. And if, as the opposition claims, it was a government attack, then a delay of days or weeks would give it enough time for forensic evidence to become controversial and for evidence of munitions used to be removed. The Syrian government insists it was the rebels who carried out the attack. 
 
 But they only have a mandate to visit three sites previously agreed between the UN and the Syrian government, including the northern town of Khan al-Assal, where some 26 people were killed in an alleged chemical attack in March. 
 
 The Syrian government has described the latest allegations as "illogical and fabricated". The Syrian army said opposition forces had made up the claims to divert attention from their recent huge losses. 
 
 Heavy shelling continued around Ghouta on Thursday, reports say. 
 
 'Red lines crossed' 
 
 A spokesman for Ban Ki-moon, Eduardo del Buey, said on Thursday that the secretary general Ban Ki-moon believed the attacks "need to be investigated without delay". 
 
 Mr Ban was sending his disarmament chief Angela Kane to Damascus to press for an investigation, he said. 
 
 Earlier, Mr Fabius told the French BFM TV channel that if the use of chemical weapons was confirmed, "France's position is that there must be a reaction, a reaction that could take the form of a reaction with force". 
 
 He did not elaborate on whether that meant backing military action, but did rule out the idea of deploying troops inside Syria. 
 
 The US state department said it had yet to "conclusively determine" what had taken place in Damascus, but that it was urgently gathering information. 
 
 Media playback is unsupported on your device Media caption US state department spokeswoman Jen Psaki: ''The president, of course, has a range of options'' 
 
 If President Bashar al-Assad's government was found to be behind a chemical weapons attack it would be "an outrageous and flagrant escalation", spokeswoman Jen Psaki said. 
 
 President Barack Obama warned last year that the use of such weapons would cross a "red line". 
 
 The British Foreign Office said earlier in a statement that the UK and 36 other countries had formally referred the latest allegations to Mr Ban, and called for inspectors "to be granted the necessary access to enable their investigation into these latest allegations as a matter of urgency". 
 
 "We believe a political solution is the best way to end the bloodshed," said the statement, but added that the UK has "said many times we cannot rule out any option... that might save innocent lives in Syria". 
 
 Turkish Foreign Minister Ahmet Davutoglu also stressed the need for an urgent response, and criticised the lack of UN action. 
 
 Image caption Video footage and photographs have been posted online appearing to show civilians, including children, suffering the effects of a chemical agent. In pictures: Damascus attacks How should the world respond? 
 
 On Wednesday, the UN Security Council failed to agree on a statement condemning the attack at an emergency meeting. 
 
 It was blocked by China and Russia, which have repeatedly backed the Syrian government since the crisis began. 
 
 Russia is supporting calls for an investigation, however, because it believes the opposition might have carried out the attack itself, as "premeditated provocation" in an attempt to win the backing of the UN. 
 
 Disturbing footage 
 
 Opposition activists said that more than 1,000 people were killed after government forces launched rockets with toxic agents into the Damascus suburbs in the Ghouta region early on Wednesday. 
 
 Chemical weapons claims Khan al-Assal, 19 March 2013 - Syrian state media accuse rebels of killing 31 people with rockets containing "chemical materials". Rebels blame the army for the attack. 
 
 - Syrian state media accuse rebels of killing 31 people with rockets containing "chemical materials". Rebels blame the army for the attack. Al-Otaybeh, 19 March 2013 - Opposition activists allege an attack in which six people are reported dead, apparently in reprisal for gains made by rebel forces. 
 
 - Opposition activists allege an attack in which six people are reported dead, apparently in reprisal for gains made by rebel forces. Adra, 24 March 2013 - The LCC activist network say two people are killed in an attack. 
 
 - The LCC activist network say two people are killed in an attack. Sheikh Maqsoud, Aleppo, 13 April 2013 - At least three people are killed in an attack; internet footage of the victims shows symptoms consistent with exposure to nerve gas. 
 
 - At least three people are killed in an attack; internet footage of the victims shows symptoms consistent with exposure to nerve gas. Saraqeb, 29 April 2013 - Eyewitnesses say canisters containing a poisonous gas are dropped from a helicopter above the town. Eight people are injured, one of whom later dies. 
 
 - Eyewitnesses say canisters containing a poisonous gas are dropped from a helicopter above the town. Eight people are injured, one of whom later dies. Ghouta, 21 August 2013 - By far the most serious alleged incident, with hundreds reported dead in attacks on the outskirts of Damascus Q&A: Damascus 'toxic attacks' Syria chemical weapons allegations Syria's chemical weapons stockpile How to investigate chemical arms allegations 
 
 The BBC has been unable to independently confirm the death toll. 
 
 Activists said Wednesday's attack took place as part of heavy government bombardment in the region surrounding Damascus, with government forces trying to drive out rebel forces. The areas affected included Irbin, Duma and Muadhamiya. 
 
 Video footage shows dozens of bodies with no visible signs of injuries, including small children, and survivors being treated in makeshift hospitals, with victims, including many children, having convulsions. 
 
 Chemical weapons experts have told the BBC that footage appears genuine and that the injuries shown are consistent with nerve agents. 
 
 While it is not clear how many died in the bombardment of the sites and how many deaths were due to any exposure to toxic substances, experts say it would be almost impossible to fake so many dead and injured including children and babies. 
 
 Both the rebels and government forces have accused each other of using chemical weapons throughout the 28-month conflict. 
 
 Syria is believed to have large undeclared stockpiles of mustard gas and sarin nerve agent. 
 
 The government has implied it has chemical weapons, but said they would not be used against civilians. 
 
 More than 100,000 people are believed to have been killed during the 28-months of conflict Syria. ||||| The foreign minister of France on Tuesday accused the Syrian government of attacking its people with chemical weapons at least 14 times since October, with the most recent instance just a few weeks ago. 
 
 Laurent Fabius made his statement the same day as a Human Rights Watch (HRW) issued a report which said is there strong evidence the Syrian regime used chlorine gas on rebel-held neighborhoods last month, dropping the canisters in crude bombs on residential areas. 
 
 Speaking to reporters in Washington, Fabius cited "credible witnesses" to the attacks, which he said included the use of chlorine gas. He said it has been difficult to acquire definitive proof because chlorine gas generally evaporates too quickly to collect samples. 
 
 Fabius described the 14 attacks since last Oct. 25 as "small-scale" and not likely to spur a Western military response. 
 
 His statement and the HRW report add to growing concerns that chemical weapons are still being used in Syria months after an international deal to remove the country’s chemical weapons was reached following a sarin gas attack that killed hundreds of civilians last August. 
 
 Under the threat of U.S. airstrikes, Assad agreed to dismantle his chemical weapons program. Currently, a joint mission by the U.N. and the international chemical weapons watchdog says that 92 percent of Syria's stockpile has been shipped out of the country to be destroyed at sea. 
 
 But Fabius said that Syrian facilities that produce chemical weapons have not been destroyed, and he accused Assad's government of not being fully forthcoming with the West about its continued ability to use toxic chemicals against opponents. 
 
 HRW said forces loyal to Assad likely used chlorine gas on three towns in northern Syria in mid-April, according to interviews with 10 witnesses, video footage and photographs. 
 
 "Evidence strongly suggests that Syrian government helicopters dropped barrel bombs embedded with cylinders of chlorine gas on three towns," the group said. "These attacks used an industrial chemical as a weapon, an act banned by the international treaty prohibiting chemical weapons that Syria joined in October 2013.” 
 
 In late April, the Organization for the Prohibition of Chemical Weapons (OPCW), a United Nations watchdog that won the Nobel Peace Prize for taking the lead to remove Syria’s stockpile last year, said it would investigate the new chlorine claims, but it has not commented further on the issue. 
 
 In one incident, the Syrian government blamed an Al-Qaeda-linked group, the Nusra Front, for using chlorine gas on civilians in the rebel-held town of Kfar Zeita. It has not commented on other attacks. An extensive Associated Press investigation in late April found consistent claims that chlorine gas had been used in Kfar Zeirta. 
 
 HRW said testimony from eye-witnesses indicated that chlorine canisters were embedded into crude explosive-laden barrels, which military helicopters dropped at the time on rebel-held areas. 
 
 In Syria, only the pro-government forces have military aircraft, not opposition fighters. And though chlorine gas canisters are widely available, HRW said their use as a weapon is prohibited under international law. 
 
 The use of chlorine gas in bombs is not very effective as a weapon to kill people. However, HRW said it appeared the Syrian military was using the chlorine to terrorize residents into believing they had been gassed, even if many of the victims were not killed.
– Another game-changer? One of the United Nations' chief investigators says evidence is building that sarin gas was used in the Syrian conflict—but by rebels, not Bashar al-Assad's regime. "Our investigators have been in neighboring countries interviewing victims, doctors, and field hospitals" and "there are strong, concrete suspicions but not yet incontrovertible proof of the use of sarin gas," said Carla Del Ponte, a member of a panel probing alleged Syrian war crimes, Reuters reports. "This was use on the part of the opposition, the rebels, not by the government authorities," she adds. The war crimes inquiry commission Del Ponte is involved with is separate from the main UN probe into chemical weapons use in Syria. Each side accused the other of using chemical weapons during fighting in Aleppo in March, while Britain and France say they have evidence that regime forces used nerve agents in attacks on rebels. In other developments: Israeli airstrikes on targets inside Syria in recent days have killed at least 42 government soldiers, the Syrian Observatory for Human Rights tells the AP. Israel today emphasized that the strikes were not meant to aid rebels, Reuters adds, but rather to block weapons from Hezbollah. The Israeli move renewed debate on whether the US should use airstrikes against the Assad regime, the New York Times reports. John McCain, who yesterday blasted President Obama's "red line" as being "written in disappearing ink," said the strikes show that Syria's air defense system may not be much of an obstacle. Opposition activists say rebel forces shot down a military helicopter last night, killing eight government troops, the AP reports. The regime's air power has long been a chief target of rebels and activists say rebels also attacked and occupied parts of an air base in the north of the country yesterday, killing the base's commander and capturing a tank unit.