WASHINGTON (REUTERS) - US Secretary of State John Kerry will "make another stab" at seeing if the United States and Russia can work together to find a political solution to end Syria's civil war when he visits Moscow this week, a senior US official said on Monday.

Mr Kerry departs for Moscow on Monday afternoon and is scheduled to see Russian President Vladimir Putin on Tuesday to discuss a wide range of issues including the Iranian and North Korean nuclear programmes, Afghanistan and US-Russian trade.

Speaking on condition of anonymity, the US official said he did not know if Washington and Moscow might be able to move forward on a political plan for Syria that they endorsed on June 30, 2012, but that has since gone nowhere.

That plan, agreed to in Geneva, was aimed at resolving through talks among all sides a civil war that has cost more than 70,000 lives, but it left open the question of Syrian President Bashar al-Assad's exact fate.

Russia says his exit from power must not be a precondition for a dialogue among Syrians to end the conflict.

"We have their formal commitment with their agreement to the Geneva Communique of June 30, 2012 but now we need to go beyond formal commitments like that to figure out if there are ways to actually build off of it," said the US official.

"It is no secret that so far we have not been able to do that but we certainly want to try to make another stab at it, to make another effort at it, because events on the ground have become steadily worse," he added.

"The casualty figures are mounting, the rate of killing has gone up and as the Israeli strikes show, the situation is adding to instability in the region," the official said.

Israel conducted two air raids on neighbouring Syria over the weekend.

The attacks hit targets manned by Mr Assad's elite troops in the Barada River valley and Qasioun Mountain, residents, activists and opposition military sources said.

They included a compound linked to Syria's chemical weapons program, air defences and Republican Guards' facilities, the sources said.

Russia said it was concerned the chances of foreign military intervention in Syria were growing, suggesting its worry stemmed in part from media reports about the alleged use of chemical weapons in the conflict.

The US official played down comments by Mr Carla Del Ponte, a member of a U.N. inquiry into war crimes and other human rights violations in Syria, that human rights investigators have gathered testimony from casualties of Syria's civil war and medical staff indicating that rebel forces have used the nerve agent sarin.

The Geneva-based inquiry is separate from an investigation of the alleged use of chemical weapons in Syria instigated by UN Secretary-General Ban Ki Moon, which has since stalled.

"Our understanding has been that the armed opposition does not have such weapons and so we'll have to re-check our facts but our initial take on that was that they do not have such things in their arsenal," said the US official, stressing that the United States takes all such reports seriously.

"We have no information to suggest that they have either the capability or the intent to deploy or use such weapons," he said of the rebel fighters.