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. ||||| Image copyright Reuters Image caption The inspectors were unable to reach the rebel-held village of Kafr Zaita, scene of several alleged attacks 
 
 A team from the world's chemical weapons watchdog believes toxic chemicals such as chlorine are being used in a "systematic manner" in Syria. 
 
 The Organisation for the Prohibition of Chemical Weapons sent a fact-finding mission to the country's north last month to investigate several incidents. 
 
 The inspectors' preliminary report says information was gathered that seemed to corroborate the testimony of medics. 
 
 But an attack on their convoy prevented them presenting definitive conclusions. 
 
 They were travelling to the rebel-held village of Kafr Zaita, the scene of at least six reported chlorine gas attacks, to obtain clinical and environmental evidence when one of their vehicles was hit by a bomb. 
 
 Their convoy was later ambushed and a vehicle was shot at from close range by gunmen who then detained them briefly. 
 
 'Denial of access' 
 
 The OPCW mission was tasked with investigating the alleged use of chlorine in more than a dozen attacks in Hama and Idlib provinces. 
 
 In each case, barrel bombs were allegedly dropped from helicopters. After they exploded, casualties reportedly began displaying symptoms typical of chlorine poisoning, including sore eyes, irritated skin, breathing difficulties and bloody foaming from the mouth. Several people died. 
 
 The mission's report concludes that information it had gathered seemed to "corroborate the testimonies provided by the treating physicians" with whom it was able to establish contact. "The similarities in the witness accounts reviewed over the course of time became conspicuous", it adds. 
 
 The attack on the team and the resulting denial of access to Kafr Zaita prevented the inspectors from presenting definitive conclusions. 
 
 "It is nonetheless the considered view of the fact-finding mission that the available information cannot be dismissed as unconnected, random, or of a nature attributable to purely political motives," the report says. 
 
 "This information lends credence to the view that toxic chemicals, most likely pulmonary irritating agents such as chlorine, have been used in a systematic manner in a number of attacks." 
 
 On Monday afternoon, the OPCW's director general, Ahmet Uzumcu, said the team would "continue its important work in gathering facts and bringing them to light". 
 
 Image copyright YouTube Image caption A video from Kafr Zaita on 11 April appears to shows yellow smoke after a barrel-bomb explosion 
 
 The BBC's Anna Holligan in The Hague says chlorine is a "dual-use chemical". It has many legitimate industrial functions, but its use as a weapon is banned by the Chemical Weapons Convention (CWC). If high concentrations of the chemical enter the lungs it can cause death. 
 
 Chlorine was not on the list of toxic agents declared by Syria as part of last year's deal for President Bashar al-Assad to hand over the military's vast chemical stockpile, our correspondent adds. 
 
 Syria has so far shipped out 92% of its stockpile of chemical weapons. However, the other 8% remains at one site and cannot be moved to the port of Latakia because of the security situation. 
 
 In a separate development on Wednesday, activist groups said government air strikes had killed at least seven people, several of them children, near Syria's southern border with Jordan. 
 
 Helicopter gunships were used in the attack on a camp for displaced civilians in Deraa province, where fighting has intensified in recent weeks, with rebels making gains, they added.
– The Syrian government and the opposition have blamed each other for reported poison gas attacks in the rebel-controlled village of Kfar Zeita; now, the opposition says it has images that point the finger at the regime. Activists released images and video of an unexploded canister marked with the chemical symbol for chlorine. They say the canister turned up in Kfar Zeita, though Reuters notes it can't verify the report. A UK-based expert isn't certain, but he tells Reuters that "It looks like (the government has) taken an industrial chlorine cylinder, put it in (an) improvised barrel bomb, and dropped it out of a helicopter." The government, for its part, is blaming an al-Qaeda-linked group, the Nusra Front. Both sides agree that chlorine gas was used. The chemical isn't among those Bashar al-Assad declared to international officials in a deal to destroy toxic weapons, Reuters notes. In other news from Syria: A 13th batch of chemical weapons was shipped today from Syria, the Organization for the Prohibition of Chemical Weapons says, per the AP. That means 65% of the declared substances have left the country. But "the frequency and volumes of deliveries have to increase significantly" if Syria is to meet a June 30 deadline to destroy the full stockpile, says the watchdog's director. Assad says the war has reached a "turning point"—"both militarily in terms of the army's achievements in the war against terror, and socially in terms of national reconciliation processes and growing awareness of the truth behind the (attacks) targeting the country," AFP notes, via the Guardian. Syrian warplane attacks yesterday on the edge of Damascus killed 13, says the Syrian Observatory for Human Rights.