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) ||||| Syrian rebels shot down a military helicopter in the country's east, killing eight government troops on board a day after opposition forces entered a sprawling military air base in the north, activists said Monday. 
 
 This citizen journalism image provided by Aleppo Media Center AMC which has been authenticated based on its contents and other AP reporting, shows planes on the ground at the Kweiras military air base... (Associated Press) 
 
 In the past months, rebels fighting to topple President Bashar Assad have frequently targeted military aircraft and air bases in an attempt to deprive his regime of a key weapon used to target opposition strongholds and reverse rebel gains in the 2-year-old conflict. 
 
 The Britain-based Syrian Observatory for Human Rights on Monday posted a video online showing several armed men standing in front of the wreckage. One of the fighters in the footage says it's a helicopter that the rebels shot down late Sunday in the eastern province of Deir el-Zour, along Syria's border with Iraq. 
 
 As the man speaks, the camera shifts to a pickup truck piled with bodies. The fighter is then heard saying that all of Assad's troops who were aboard the helicopter were killed in the downing. He says Islamic fighters of the Abu Bakr Sadiqq brigade brought down the helicopter as it was taking off from a nearby air base in the provincial capital of Deir el-Zour. 
 
 The Observatory, which relies on a network of activists on the ground, said eight troops were killed. 
 
 On Sunday, rebels occupied parts of a military air base in northern Syria after days of fighting with government troops who were defending the sprawling facility near the border with Turkey for months, the Observatory said. 
 
 Assad's warplanes were pounding rebel positions inside the Mannagh air base Monday as clashes between rebels and government forces raged on, the Observatory said, adding there was an unknown number of casualties on both sides. 
 
 The day before, rebels moved deep into the air base despite fire from government warplanes, capturing a tank unit inside the base and killing the base's commander, Brig. Gen. Ali Salim Mahmoud, according to another activists group, the Aleppo Media Center. 
 
 The fighting came hours after Israeli warplanes struck areas in and around the Syrian capital, setting off a series of explosions as they targeted a shipment of highly accurate, Iranian-made guided missiles believed to be bound for Lebanon's Hezbollah militant group, officials and activists said. 
 
 The attack, the second in three days and the third this year, signaled a sharp escalation of Israel's involvement in Syria's civil war. Syrian state media reported that Israeli missiles on Sunday struck a military and scientific research center near Damascus and caused casualties. 
 
 The Syrian conflict started with largely peaceful protests against President Bashar Assad's regime in March 2011, but eventually turned into a civil war that has killed more than 70,000 people according to the United Nations. 
 
 More than one million Syrians have fled their homes during the fighting and sought shelter in the neighboring countries such as Jordan, Lebanon and Turkey. Millions of others have been displaced inside Syria. ||||| Israeli public security minister Tzaji Hanegbi speaks at a news conference in Mexico City March 25, 2004. Hanegbi is in Mexico to speak with local and federal police chiefs. 
 
 JERUSALEM Israel sought to persuade Syrian President Bashar al-Assad on Monday that its recent air strikes around Damascus did not aim to weaken him in the face of a more than two-year-old rebellion. 
 
 Officials say Israel is reluctant to take sides in Syria's civil war for fear its actions would boost Islamists who are even more hostile to Israel than the Assad family, which has maintained a stable stand off with the Jewish state for decades. 
 
 But Israel has repeatedly warned it will not let Assad's ally Hezbollah receive hi-tech weaponry. Intelligence sources said Israel attacked Iranian-supplied missiles stored near the Syrian capital on Friday and Sunday that were awaiting transfer to Hezbollah guerrilla group in neighboring Lebanon. 
 
 Syria accused Israel of belligerence meant to shore up the outgunned anti-Assad rebels - drawing a denial on Monday from veteran Israeli lawmaker Tzachi Hanegbi, a confidant of Prime Minister Benjamin Netanyahu. 
 
 Interviewed on Israel Radio, Hanegbi said the Netanyahu government aimed to avoid "an increase in tension with Syria by making clear that if there is activity, it is only against Hezbollah, not against the Syrian regime". 
 
 Hanegbi noted Israel had not formally acknowledged carrying out the raids in an effort to allow Assad to save face, adding that Netanyahu began a scheduled visit to China on Sunday to signal the sense of business as usual. 
 
 "DIPLOMATIC CHANNELS" 
 
 The Assad government has condemned the air strikes as tantamount to a "declaration of war" and threatened unspecified retaliation. 
 
 But Hanegbi said Israel was ready for any development if the Syrians misinterpreted its messages and was ready "to respond harshly if indeed there is aggression against us". 
 
 As a precaution, Israel deployed two of its five Iron Dome rocket interceptors near the Syrian and Lebanese fronts and grounded civilian aircraft in the area, although an Israeli military spokesman said the airspace would reopen on Monday. 
 
 Yedioth Ahronoth, Israel's biggest-selling newspaper, said the Netanyahu government had informed Assad through diplomatic channels that it did not intend to meddle in Syria's civil war. 
 
 Israeli officials did not immediately confirm the report, but one suggested that such indirect contacts were not required. 
 
 "Given the public remarks being made by senior Israeli figures to reassure Assad, it's pretty clear what the message is," the official told Reuters on condition of anonymity. 
 
 Military analysts say Syria would be no match for Israel, a U.S. defense ally, in any confrontation. But Damascus, with its leverage over Hezbollah, could still consider proxy attacks through Lebanon, where Israel's conventional forces fought an inconclusive war against the Iranian-backed guerrillas in 2006. 
 
 Tehran, which has long backed Assad, whose Alawite minority has religious ties to Shi'ite Islam, denied Israel's attack was on arms. Shi'ite Hezbollah did not comment. 
 
 (Writing by Dan Williams; Editing by Jon Boyle) ||||| FILE -- In this Thursday, Nov. 15, 2012 file photo, the Iron Dome defense system fires to intercept an incoming missiles from Gaza in the port town of Ashdod, Israel. Israel's military has deployed Iron... (Associated Press) 
 
 This photo released by the Syrian official news agency SANA, shows a general view of damaged buildings wrecked by an Israeli airstrike, in Damascus, Syria, Sunday, May 5, 2013. Israeli warplanes struck... (Associated Press) 
 
 FILE - In this Saturday, Nov. 17, 2012 file photo, an Israeli Iron Dome missile is launched near the city of Be'er Sheva, southern Israel, to intercept a rocket fired from Gaza. Israel's military has... (Associated Press) 
 
 This photo released by the Syrian official news agency SANA, shows damaged buildings wrecked by an Israeli airstrike in Damascus, Syria, Sunday, May 5, 2013. Israeli warplanes struck areas in and around... (Associated Press) 
 
 This photo released by the Syrian official news agency SANA shows a damaged building wrecked by an Israeli airstrike in Damascus, Syria, Sunday, May 5, 2013. Israeli warplanes struck areas in and around... (Associated Press) 
 
 This photo released by the Syrian official news agency SANA shows a general view of damaged buildings wrecked by an Israeli airstrike, in Damascus, Syria, Sunday, May 5, 2013. Israeli warplanes struck... (Associated Press) 
 
 In this image taken from video obtained from the Ugarit News, which has been authenticated based on its contents and other AP reporting, a Israeli airstrikes hit Damascus, Syria, early Sunday, May 5,... (Associated Press) 
 
 In this image taken from video obtained from Shaam News Network, which has been authenticated based on its contents and other AP reporting, smoke and fire fill the skyline over Damascus, Syria, early... (Associated Press) 
 
 In this image taken from video obtained from the Ugarit News, which has been authenticated based on its contents and other AP reporting, smoke and fire fill the the skyline over Damascus, Syria, early... (Associated Press) 
 
 In this photo released by the Syrian official news agency SANA, damaged buildings wrecked by an Israeli airstrike are seen in Damascus, Syria, Sunday, May 5, 2013. Israeli warplanes struck areas in and... (Associated Press) 
 
 In this photo released by the Syrian official news agency SANA, damaged buildings wrecked by an Israeli airstrike are seen in Damascus, Syria, Sunday, May 5, 2013. Israeli warplanes struck areas in and... (Associated Press)
– 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.