In this Sunday, May 5, 2013 image taken from video obtained from the Shaam News Network, which has been authenticated based on its contents and other AP reporting, men stand near a wrecked helicopter, left, in Deir el-Zour, Syria. 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. In this Sunday, May 5, 2013 image taken from video obtained from the Shaam News Network, which has been authenticated based on its contents and other AP reporting, armed men stand near the wreckage of a military helicopter, left, in Deir el-Zour, Syria. The attack was a welcome victory for rebels fighting to oust Assad as the two sides remain locked in stalemate in the more than 2-year-old conflict, with neither side able to deliver a decisive blow in the battlefield. In Geneva, Carla Del Ponte, a member of a U.N. commission probing alleged war crimes and other abuses in Syria, said there were indications that Syrian rebels have used the nerve agent sarin, but not the regime. The U.N. panel quickly distanced itself from Del Ponte's claim, saying it has no conclusive evidence about the alleged use of sarin as chemical weapons. Senior U.S. administration officials also said the Assad regime retains custody of all the chemical weapons stockpiles it is believed to have. The officials, who spoke on condition of anonymity because they were not authorized to speak publicly on the issues, said they believe the armed opposition in Syria does not have chemical weapons and therefore wouldn't be able to launch a sarin attack. The dueling statements highlighted the difficulties of investigating allegations of chemical weapons use. President Barack Obama has said the use of chemical weapons by the regime is a "red line" but he needs more time to determine if Assad's forces had used chemical weapons in the Syria's civil war. Del Ponte, a former war crimes prosecutor, told Italian-language Swiss public broadcaster SRI that her panel's investigators have "strong, concrete suspicions but not yet incontrovertible proof of the use of sarin gas from the way the victims were treated." We have evidence on the use of chemical weapons, in particular sarin. Not by the government, but the opposition," Del Ponte said in the interview that was broadcast Sunday night. She said this is based on interviews with victims, doctors and field hospitals in neighboring countries. On Monday, the commission issued a statement saying it "wishes to clarify that it has not reached conclusive findings as to the use of chemical weapons in Syria by any parties to the conflict." As a result, "the commission is not in a position to further comment on the allegations at this time." The four-member panel was appointed by the 47-nation Human Rights Council, the U.N.'s top human rights body, to gather evidence on suspected war crimes and other abuses. It began its investigation in August 2011. It has had almost no access to Syria, though earlier this year it said it had conducted at least 1,500 interviews and exhaustively corroborated its findings with other sources. Fighting in Syria continued unabated on Monday as Assad's warplanes pounded rebel positions inside the Mannagh air base in the north and government troops regained control of six villages along the strategic road that links the northern city of Aleppo with its civilian airport, the country's second largest. Also Monday, the Britain-based Syrian Observatory posted a video showing several armed men standing in front of wreckage that one of the fighters says is a helicopter shot down late Sunday in the eastern province of Deir el-Zour, along Syria's border with Iraq. The video was in line with Associated Press reporting in the area. The Syrian government did not comment. In the past months, rebels have frequently targeted military aircraft and air bases in an attempt to deprive the regime of a key weapon used to target opposition strongholds and reverse rebel gains. The rebels occupied parts of the Mannagh military air base on Sunday after weeks of fighting with government troops who have been defending the sprawling facility near the border with Turkey for months, the Observatory said. Clashed raged inside the base Monday and the Observatory said both sides suffered casualties in the fighting. Much of the north has been in rebel hands since opposition fighters launched an offensive in the area last summer, capturing army bases and large swaths of land along the border with Turkey and whole neighborhoods inside Aleppo, Syria's largest city. But last month government troops recaptured the village of Aziza on a strategic road that links Aleppo with its airport and military bases, dealing a huge setback to the rebels unable to hold on to the territory in the face of Assad's superior air power. The state-run news agency SANA reported Monday that "armed forces restored security and stability to (six) villages" south of the city and along the airport highway, calling it a "major strategic victory in the north." The Syrian conflict started with largely peaceful protests against Assad's regime in March 2011, but eventually turned into a civil war that the U.N. says has killed more than 70,000 people. Associated Press writers John Heilprin in Geneva and Bradley Klapper and Josh Lederman in Washington contributed to this report.