US Secretary of State Hillary Clinton accused Russia Tuesday of sending attack helicopters to Syria, saying Moscow is lying about its arms shipments.

"We are concerned about the latest information we have that there are attack helicopters on the way from Russia to Syria, which will escalate the conflict quite dramatically," Clinton told a think-tank discussion in Washington.

"There's no doubt the onslaught continues the use of heavy artillery and the like. We have confronted the Russians about stopping their continued arms shipments to Syria," she told the discussion at the Brookings Institution.

"They have from time to time said we shouldn't worry, that everything they're shipping is unrelated to their actions internally. That's patently untrue."

Clinton pointed out that UN-Arab League envoy Kofi Annan was now trying to put together a contact group to work on a road map for a political transition that would see Syrian President Bashar al-Assad step aside.

The group would "include Russia," Clinton said, adding the United States was in agreement with that.

"Russia has increasingly said that it was not defending Assad, but it worried about what came after Assad and that it would work on political transition," Clinton said.

"But there are always a lot of caveats that they then interpose," she said.