BEIRUT — 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 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 Assad’s government 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. Activists said they had gathered their information from sources in Syrian military hospitals.
The Syrian government has not said how many people it believes were killed in the airstrike, but casualty counts given by state media have been much lower.
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 the United States would intervene only if Syria has used chemical weapons or if such use is imminent.
But there have been separate claims that chemical weapons were used by the insurgents, who are supported by many Western and Arab nations.
In a weekend interview with Swiss-Italian television, Carla Del Ponte, one of the leading figures in a Geneva-based UN commission of inquiry, said there were strong suspicions that the rebels seeking Assad’s overthrow had themselves used sarin, a nerve agent, but there was no “incontrovertible proof” that they had.
Del Ponte did not elaborate on where the chemicals might have been used. She is one of four investigators chosen by the 47-nation UN Human Rights Council in August 2011 to report periodically on the Syrian situation. Their next report is scheduled for publication in late May, officials in Geneva said, but it was not immediately clear whether it would document findings relating to chemical weapons.
George Little, a Pentagon spokesman, declined to comment on Del Ponte’s remarks directly but 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.”
Rebel spokesmen said the reported findings described by Del Ponte were false.
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 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 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.
Benjamin Netanyahu, the prime minister of Israel, has declared that the delivery of ‘‘game-changing’’ weapons to Hezbollah, the Lebanon-based terror group, could trigger Israeli military intervention.
The first test of this policy came in January when an Israeli airstrike in Syria destroyed a shipment of advanced antiaircraft missiles bound for Hezbollah, according to US officials.
Israel and Hezbollah fought an inconclusive monthlong war in 2006 and are bitter enemies. During that war, sparked by a Hezbollah cross-border raid, the militant group fired some 4,000 rockets into Israel.
When Israeli intelligence determined last week that Iranian-made Fateh-110 missiles had entered Syria, the military prepared to strike again.
Although Israel has not officially confirmed the operation, the Associated Press quoted a senior official as saying a first airstrike at a Damascus airport early Friday destroyed most of the shipment, while a series of subsequent airstrikes on nearby locations Sunday took out the remnants of the missiles.
Syria and Iran have hinted at retaliation for the Israeli raids, though they took no action Monday and the official rhetoric has been mild.
There also were no new reports of Israeli airstrikes in Syria.