The United States said Monday it was "highly skeptical" of an assertion that Syrian rebels had used chemical weapons, after a UN human rights investigator suggested the opposition had deployed sarin gas.

Washington also reiterated that Israel had every right to protect itself and to prevent sophisticated weaponry from getting into the hands of Hezbollah, following several Israeli raids on sites in Damascus.

"We are highly skeptical of suggestions that the opposition could have or did use chemical weapons," said White House spokesman Jay Carney.

"We find it highly likely that any chemical weapon use that has taken place in Syria was done by the Assad regime. And that remains our position," Carney said.

A senior US official separately said that Washington had no information to suggest Syrian rebels had "capability or the intent to deploy or use such weapons."

Carla del Ponte, a former war crimes prosecutor, told Swiss public broadcaster RSI that "according to the testimonies we have gathered, the rebels have used chemical weapons, making use of sarin gas."

The senior official also noted that "Ms Del Ponte does not work on the same expert team, which the United Nations has assembled to go into Syria as they are two different parts of the United Nations organization."

Carney also reiterated President Barack Obama's statement at the weekend that Israel was within its rights to safeguard its security, though would not directly comment on Israeli military reaction.

"Israel certainly has the right to be concerned about the transfer of sophisticated weapons to Hezbollah, and that has been a concern of Israel's for a long time," Carney said.

The air strikes early on Sunday near Damascus were the Jewish state's second reported air raids on Syria in 48 hours. An early Friday raid had targeted a weapons storage facility at Damascus airport.

A watchdog group, the Syrian Observatory for Human Rights told AFP that 42 soldiers were killed in the strikes and another 100 remained unaccounted for.

Carney also sidestepped a question about whether the slaughter of civilians in a civil war that has claimed more than 70,000 lives amounted to genocide.

"The terminology that may be used by courts or the United Nations or others, I will leave to them," Carney said.

"But it is heinous and despicable. It is the kind of action that long ago rendered Assad incapable of continuing in power with any kind of legitimacy."

Obama is coming under increasing pressure in Washington from domestic political critics who are demanding action, including an operation to arm rebels or set up a no-fly zone to protect civilians.

The White House has said it is considering all its options, including reviewing its opposition to date to providing weapons to rebels, but Carney declined to provide a timeline for the deliberations.