WASHINGTON -- Secretary of State Hillary Rodham Clinton charged Tuesday that Russia is sending military attack helicopters to Syria, and warned that the move could escalate the 15-month-old conflict “quite dramatically.”

Russia has denied that President Bashar Assad’s regime is using Russian weapons against its political opponents, but Clinton derided that claim as untrue.

"We have confronted the Russians about stopping their continued arms shipments to Syria,” Clinton said during an appearance at the Brookings Institution think tank. “They have, from time to time, said that we shouldn't worry -- everything they are shipping is unrelated to [the Syrian government's] actions internally. That's patently untrue."

"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.

Victoria Nuland, the State Department spokeswoman, declined to disclose the source of Clinton’s information or to elaborate on it.

A Pentagon spokesman, Capt. John Kirby, said he couldn’t confirm Clinton’s information but added, “We know that they are using helicopter gunships now to attack their own people.”

New shipments of Russian attack helicopters, if true, would be a setback for international efforts to scale back a conflict that already has claimed more than 10,000 lives and appears on the verge of civil war. Clinton’s charge is likely to sharpen frictions between the United States and Russia. The Obama administration is seeking Russian diplomatic help on a number of fronts.

The next round of talks on Iran’s disputed nuclear program are scheduled to begin Monday in Moscow, and U.S. and European diplomats are looking to Russia to use its influence with the Iranians during that session.

President Obama is expected to meet in private with Russian President Vladimir Putin at a gathering of the Group of 20 industrial nations that also begins Monday in Mexico.

U.S. officials have warned in recent days about the growing violence in Syria. Clinton said government forces have massed around Aleppo, Syria’s financial center.

“We are watching this very carefully,” she said.

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-- Paul Richter

Photo: A Syrian revolutionary flag waves on top of a building on the outskirts of Aleppo, Syria, on June 12, 2012. Credit: Associated Press

