Russia dismissed on Wednesday claims by U.S. Secretary of State Hillary Clinton that it was selling attack helicopters to Syria and accused the United States of arming rebels fighting against the rule of Syrian President Bashar al-Assad.

“We are completing right now the implementation of contracts that were signed and paid for a long time ago,” Foreign Minister Sergei Lavrov said after talks in the Iranian capital of Tehran. “All these contracts concern exclusively anti-aircraft defense.”

“We are not delivering to Syria, or anywhere else, items that could be used against peaceful demonstrators,” Lavrov went on. “In this we differ from the United States, which regularly delivers riot control equipment to the region, including a recent delivery to a Persian Gulf country. But for some reason the Americans consider this to be fine."

And Lavrov, speaking on Iranian state television, also said the United States was "providing arms and weapons to the Syrian opposition that can be used in fighting against the Damascus government."

Russia's top diplomat's comments came a day after Clinton told a forum in Washington that Moscow’s repeated assurances that the arms it supplies to Syria could not be used to attack protesters in the Middle East country were “patently untrue.”

"We are concerned about the latest information we have that there are attack helicopters on the way from Russia to Syria,” she added, without giving further details.

Clinton also warned that such supplies would “escalate the conflict quite dramatically."

"We know that the Assad regime is using helicopter gunships against their own people," Pentagon spokesman Captain John Kirby said later. He also said, however, that he had no information on a new shipment of attack helicopters from Russia to Syria.

Syria is one of Russia’s major weapons clients, and Moscow has opposed proposals for an arms embargo on Damascus, saying this would give rebel forces an unfair advantage in the conflict.

Russia – along with China – has also twice vetoed UN resolutions against Damascus over what it says is a pro-rebel bias. Moscow has, however, fully backed UN envoy Kofi Annan’s faltering peace plan for Syria.

And Lavrov repeated again on Wednesday Moscow's assertion that its stance was not based on support for Assad, who rules Russia's sole remaining ally in the Arab world.

"Our position is not based on support for Bashar al-Assad or anyone else," he said. "We do not want to see Syria disintegrate."

Russian military experts suggested on Tuesday that Moscow may be repairing earlier-supplied helicopters for Syria, rather than providing Damascus with new models.

“There were large-scale deliveries of attack helicopters to Syria in the Soviet era,” said Andrei Frolov, editor of the Arms Exports research journal. “The last deliveries of Russian helicopters took place at the start of the 1990s.”

“There is no information about new contracts for the delivery of attack helicopters,” he went on. “This might be a case of the repair or possible modernization of earlier delivered machines.”

The editor of the Moscow Defense journal, Mikhail Barabanov, said the helicopters possibly being repaired in Russia might be Soviet-era “Mi-24 or Mi-17” models.

Clinton was answering a question on the Pentagon’s purchase of Russian helicopters for the Afghan military ahead of a pull-out by U.S. troops.

U.S. Senator John Cornyn, a member of the Senate Armed Services Committee, sent a letter to Defense Secretary Leon Panetta earlier this week calling Rosoboronexport "an enabler of mass murder in Syria” and called for sanctions against the company.

Pentagon spokesman George Little said Washington “understood” Cornyn’s concerns, but said the helicopters involved were the best option to allow Afghan forces to “take on their own fights inside their own country."

The UN says at least 9,000 people have been killed in Syria since the uprising against Assad began in March 2011, but is unable to update its figures. Syrian activists say the true number of those killed is at least 13,000.

But Syria hit out on Wednesday at comments by U.N.'s peacekeeping chief Herve Ladsous, who said the crisis in the country had now descended into “civil war.”

The Syria Foreign Ministry said in a statement that its forces were fighting armed groups responsible for "killings, kidnappings and other terrorist acts."