"We do NOT -- repeat NOT -- provide arms to anyone in Syria," State Department spokesman Mark Toner wrote in an e-mail to FoxNews.com.
The State Department said all U.S. aid is strictly "non-lethal," including humanitarian assistance, and communications and medical equipment.
The Obama administration Tuesday accused Russia of planning to supply new attack helicopters to Syria and contradicted Moscow's claim it was not supporting President Bashar Assad's violent crackdown on dissents.
"We strongly believe that arms shipments to the Assad regime are the wrong thing to be doing and that that only heightens the violence against the Syrian people," White House spokesman Jay Carney said Wednesday during the daily press briefing. "And it's part of a broader perspective that we have that we've made very clear that we do not believe that any kind of support for the Assad regime right now is helpful because of what Assad is doing to his people."
Later he added: "We believe that everyone should stop providing -- everyone who is providing weapons to the Assad regime should halt the provision of those weapons because it only -- as we've said, providing more weapons only further militarizes the situation."
Secretary of State Hillary Clinton said in Washington at an appearance with Israeli President Shimon Peres Tuesday the United States is "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."
Moscow, a longtime Assad ally and arms supplier, hadn't responded to Clinton's remarks. Russia insists it provides Damascus only with weapons that can be used in self-defense.
Assad claims his forces are defending the country against "foreign-backed terrorists."
Pentagon spokesman Capt. John Kirby said he could not confirm Clinton's assertion but said the Assad regime was already using attack helicopters against the opposition.
The State Department said Clinton was referring to new helicopter deliveries on their way to Syria.
Russian state-controlled arms trader JSC Rosoboronexport intends to fulfill its contract to supply arms to Syria, the agency's No. 2 official said.
"No one can ever accuse Russia of violating the rules of armaments trade set by the international community," Deputy Chief Executive Officer Igor Sevastyanov said in Paris in remarks carried by Russia's RIA Novosti news agency Wednesday.
"The contract was signed long ago and we supply armaments that are self-defense rather than attack weapons, and there can be no talk about any violations by Russia or Rosoboronexport either de jure or de facto," he said in response to a question about whether Russia would continue to carry out its contract to supply Pantsir-S1 short- to medium-range surface-to-air missiles and anti-aircraft artillery weapon systems to Syria.
"De jure" and "de facto" are used instead of "in law" and "in practice" when describing political or legal situations.
The State Department said Monday Assad may be preparing a fifth civilian massacre in an opposition stronghold in less than three weeks, alleging "a large number of civilians" were trapped in the town of Haffa in the western Syrian Latakia province near Turkey.