WASHINGTON (Reuters) -- The United States is worried Russia may be sending Syria attack helicopters and views Russian claims that its arms transfers to Syria are unrelated to the conflict there as "patently untrue," US Secretary of State Hillary Clinton said Tuesday.

"We have confronted the Russians about stopping their continued arms shipments to Syria. They have, from time to time, said that we shouldn't worry -- everything they are shipping is unrelated to their (the Syrian government's) actions internally. That's patently untrue," Clinton said at an appearance organized by a think tank.

"And 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," she said.