(CNN) -- Secretary of State Hillary Clinton accused Russia of sending attack helicopters to Syria.
At a Washington think tank on Tuesday, Clinton said, "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 said the United States has repeatedly asked Russia to stop arms shipments to President Bashar al-Assad's regime, but Russia has said that anything it is sending is not being used against the public unrest.
"That's patently untrue," Clinton said before making the accusation about the helicopter shipments.
Clinton's comments were made at the Brookings Institution, where she was speaking alongside Israeli President Shimon Peres.
Intelligence officials refused to comment on Clinton's assertion. The Russian Embassy in Washington also would not comment.
But the Russian state-controlled arms trader Rosoboronexport said the firm will fulfill its arms contract with Syria, the state-owned RIA Novosti news agency said.
"No one can ever accuse Russia of violating the rules of armaments trade set by the international community," Rosoboronexport Deputy CEO Igor Sevastyanov said when asked about Russia's supply of mobile gun and missile air defense systems to Syria, RIA Novosti said.
Sevastyanov added, "The contract was signed long ago, and we supply armaments that are self-defense rather than attack weapons."
Although a Pentagon spokesman was unaware of a Russian helicopter shipment to Syria, Capt. John Kirby told reporters Tuesday that al-Assad forces have been using helicopter gunships against their own people, calling the attacks "intolerable, unacceptable and just further evidence of the degree to which they are willing to kill their own people for twisted ends."
On Monday, State Department spokeswoman Victoria Nuland called the helicopter attacks on civilians a "new horrific tactic" that "constitutes a very serious escalation" of the conflict.
Russia has been the long-time principal supplier of arms to Syria since the days when it was the Soviet Union. The weapons sales have more than doubled in recent years. According to Congressional Research Service, Russia sold Syria $4.7 billion in arms from 2007 to 2010, compared with $2.1 billion from 2003 to 2006.
Nuland said the international community continues to press Russia to discontinue the weapons sales. "We are all making the point in the international community that they are on the wrong side with regard to this -- this set of issues. We've had people in Moscow in the last week making these points again and we will continue to do so," Nuland said.
Earlier this month, Clinton said the continued supply of arms from Russia has strengthened the al-Assad regime, despite denials by Russian President Vladimir Putin that any munitions it was providing to Syria were being used against its own people.
The Obama administration is also being criticized for its relationship with a Russian arms broker that is a major supplier of weapons to Syria.
Sen. John Cornyn, R-Texas, sent a letter to Defense Secretary Leon Panetta on Monday, criticizing the Department of Defense's decision to award an Army contract to Rosoboronexport to buy helicopters for the Afghanistan military. "I remain deeply troubled that the DoD would knowingly do business with a firm that has enabled mass atrocities in Syria," Cornyn wrote. "Such actions by Rosoboronexport warrant the renewal of U.S. sanctions against it, not a billion dollar DoD contract." Cornyn asked for an audit of the Pentagon's contract with the Russian company.
Human Rights First also issued a statement saying the Army contract with Rosoboronexport is "out of step" with U.S. policy to stop atrocities in Syria and called on the American government to bar any U.S. entities, including contractors, from doing business with the company.
CNN's Barbara Starr, Jill Dougherty and Phil Black contributed to this report.