BEIRUT—Syrian rebels shot down a military helicopter in the country's east, killing eight government troops on board as President Bashar Assad's troops battled opposition forces inside a sprawling military air base in the north for the second straight day, activists said Monday.

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 civil war, with neither side able to deliver a decisive blow.

In Geneva, Carla Del Ponte, a member of a UN commission probing alleged war crimes and other abuses in Syria, said there were indications that Syrian rebels were behind recent alleged chemical weapons attacks and not the regime, but the panel quickly distanced itself from the claim, saying it has no conclusive evidence about who was to blame.

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 fight.

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Del Ponte, a former war crimes prosecutor, told Italian-language Swiss public broadcaster SRI in an interview broadcast Sunday night that the indications are based on interviews with victims, doctors and field hospitals in neighbouring countries.

The four-member 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,” Del Ponte said.

The panel itself, however, issued a statement Monday 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.”

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The Observatory 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 Saddiq brigade brought down the helicopter as it was taking off from a nearby air base in the provincial capital of Deir el-Zour.

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