BEIRUT, Lebanon -- Syrian rebels said Monday that they had shot down a government helicopter in the east of the country, killing eight security troops, as new accusations emerged that insurgents seeking to overthrow President Bashar al-Assad may have used an illegal nerve agent in the country's grinding civil war. The rebels denied the assertion.
The latest battlefield accounts, focusing on the east and north of the country, came after Mr. Assad's government publicly rebuked Israel for an air attack on military targets near Damascus, the capital, early Sunday, saying the strike "opened the door to all possibilities," deepening apprehension that the civil war could spill beyond Syria's frontiers.
In the first public tally of casualties in the attack, opposition activists said Monday that 42 Syrian soldiers had died, according to The Associated Press. The activists said they had gathered their information from sources in Syrian military hospitals. The government has not said how many people it believes were killed.
Before the Israeli attack, a key question defining outside attitudes about the more than two-year-old conflict was whether chemical weapons had been used, which could draw Western powers more directly into the war. President Obama has said he would intervene only if Syria has used chemical weapons or if such use is imminent.
But there have been separate claims that chemical weapons have been used by the insurgents, who are supported by many Western and some Arab nations.
In an interview over the weekend with Swiss-Italian television, Carla Del Ponte, one of the leading figures in a Geneva-based United Nations commission of inquiry, said there were strong suspicions that the rebels seeking Mr. Assad's overthrow had themselves used sarin, a nerve agent, but there was no "incontrovertible proof" that they had.
"Our investigators have been in neighboring countries interviewing victims, doctors and field hospitals," she said, 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."
"This was use on the part of the opposition, the rebels, not by the government authorities," she added. She did not elaborate on where the chemicals might have been used. Ms. Del Ponte is one of four investigators chosen by the 47-nation United Nations Human Rights Council in August 2011 to report periodically on the Syrian situation. Their next report is set for publication in late May, officials in Geneva said, but it was not immediately clear whether it would document findings relating to chemical weapons.
It was also unclear whether Ms. Del Ponte was speaking on behalf of all four investigators. But a statement that the commission of inquiry released later in Geneva qualified Ms. Del Ponte's assertions, emphasizing "that it has not reached conclusive findings as to the use of chemical weapons in Syria by any parties to the conflict."
The United States also cast some doubt on Ms. Del Ponte's assertions. George Little, a Pentagon spokesman, declined to comment on them directly, but he told reporters in Washington: "It's our very strong belief, based on what we know, that at this stage, if chemical weapons were used, the Syrian regime would be responsible."
The insurgents said the reported findings described by Ms. Del Ponte were a "big lie."
Ms. Del Ponte's comments were made at a time when the rebels assert that Syria has already crossed the "red line" laid down as a warning to Mr. Assad not to deploy such weapons, although Western powers say the evidence is not yet conclusive.
"The Syrian regime has used the chemical weapons against civilians many times," most recently near Idlib in the north, said Maj. Gen. Adnan Sillo, a defector from the Syrian military who had headed a chemical warfare unit. "And there is no doubt that the regime will use it more often, as this is its strategy in the war since the beginning of oppressing the uprising, to move gradually."
The dispute over chemical weapons came as the Syrian Observatory for Human Rights, an opposition group that is based in Britain and draws information from a network of activists within Syria, posted video on Monday showing combatants standing in front of what appeared to be the wreckage of a helicopter.
The group said eight government soldiers were aboard the helicopter when it came down in eastern Syria. The assertion was significant because air power has given Mr. Assad's forces a significant edge, prompting the insurgents to take action against both aircraft and air bases.
On Sunday, the Observatory said, rebel forces occupied part of the Mannagh military air base in northern Syria near the border with Turkey after days of clashes, leading to renewed airstrikes by government forces seeking to dislodge them.
Anne Barnard reported from Beirut, and Alan Cowell from London. Reporting was contributed by Karam Shoumali from Antakya, Turkey, and Nick Cumming-Bruce from Geneva.
This article originally appeared in The New York Times.