* Clinton says helicopter sale would escalate conflict
* Syria conflict is civil war, UN official says
* Pentagon buys helicopters from Russian arms exporter (Adds Pentagon on defensive)
WASHINGTON, June 12 (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," U.S. Secretary of State Hillary Clinton said on Tuesday.
The comments came as the Pentagon found itself on the defensive for doing business with Russian state arms exporter Rosoboronexport, given concerns in Congress about the firm's role in arming the Syrian regime.
The 15-month-old conflict in Syria has grown into a full-scale civil war, the U.N. peacekeeping chief said on Tuesday.
Many hundreds of people, including civilians, rebels and members of President Bashar al-Assad's army and security forces have been killed since a ceasefire deal brokered two months ago was meant to halt the bloodshed.
"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," Clinton said, addressing a forum in Washington.
"That's patently untrue."
Clinton did not offer any details about the source of her information about Russia's possible shipment of attack helicopters to Syria, saying only: "We are concerned about the latest information we have that there are attack helicopters on the way from Russia to Syria."
She said such a sale "will escalate the conflict quite dramatically."
State Department spokeswoman Victoria Nuland told reporters that Clinton was concerned about helicopters now en route to Syria and not about possible past sales of Russian-origin attack helicopters to Syria.
She said that she could not elaborate or speculate on the source of Clinton's information.
Russia and China are Assad's principal defenders on the diplomatic front and, as permanent members of the U.N. Security Council with the power to veto resolutions, have stymied efforts by Western powers to condemn or call for the removal of Assad.
The United Nations says Assad's forces have killed more than 10,000 people since the uprising against his family's four-decade rule of Syria broke out in March 2011.
Pentagon spokesman Captain John Kirby said he had no knowledge of a new helicopter shipment but acknowledged that Assad's regime was turning to helicopters to stage attacks.
"We know that the Assad regime is using helicopter gunships against their own people," Kirby said.
Asked whether Russia's resupply of military equipment to Syria was enabling the Syrian armed forces to continue the killings, Kirby said: "To the degree that the Syrian armed forces use that resupply to kill their own people, then yes."
RUSSIA AND THE AFGHAN WAR
The Syrian government's use of Russian-made arms has thrown a spotlight on the Pentagon's purchase of Russian helicopters for the Afghan military, which the United States is building up so that it can take over security as American troops withdraw.
This week, U.S. Senator John Cornyn, a member of the Senate Armed Services Committee, sent a letter to Defense Secretary Leon Panetta branding Russian export firm Rosoboronexport "an enabler of mass murder in Syria."
"I remain deeply troubled that the (Pentagon) would knowingly do business with a firm that has enabled mass atrocities," Cornyn wrote. "Such actions by Rosoboronexport warrant th e renewal of U.S. sanctions against it, not a billion-dollar (Pentagon) contract."
But the Pentagon said dealing with Rosoboronexport was the only legal way to supply the helicopters to Afghanistan and attempted to differentiate between the two conflicts.
"We understand the concerns. We're not ignoring them," said Pentagon spokesman George Little. "But I would make the point that, in the case of Afghanistan, the Mi-17 is about giving them what they need and what they can use effectively to take on their own fights inside their own country."
The Pentagon's Kirby dismissed concerns that U.S. reliance on ground supply routes through Russia hampered its ability to speak out over arms shipments to Syria. But at the same time, he repeatedly stressed the need to blame Assad for the atrocities, as opposed to overly focusing on weapons suppliers.
"The focus really needs to be more on what the Assad regime is doing to its own people than the cabinets and the closets to which they turn to pull stuff out," he said. "It's really about what they're doing with what they've got in their hands."
In a March letter to Cornyn, Under Secretary of Defense for Policy James Miller acknowledged that "Rosoboronexport continues to supply weapons and ammunition to the Assad regime and ... there is evidence that some of these arms are being used by Syrian forces against Syria's civilian population." (Reporting by Arshad Mohammed and Phil Stewart; Editing by Sandra Maler and Cynthia Osterman)