With evidence that powerful new weapons are flowing to both the Syrian government and opposition fighters, the bloody uprising in Syria has thrust the Obama administration into an increasingly difficult position as the conflict shows signs of mutating into a full-fledged civil war.

Secretary of State Hillary Rodham Clinton said on Tuesday that the United States believed that Russia was shipping attack helicopters to Syria that President Bashar al-Assad could use to escalate his government’s deadly crackdown on civilians and the militias battling his rule. Her comments reflected rising frustration with Russia, which has continued to supply weapons to its major Middle Eastern ally despite an international outcry over the government’s brutal crackdown.

“We have confronted the Russians about stopping their continued arms shipments to Syria,” Mrs. Clinton said at an appearance with President Shimon Peres of Israel. “They have, from time to time, said that we shouldn’t worry; everything they’re shipping is unrelated to their actions internally. That’s patently untrue.”

Russia insists that it provides Damascus only with weapons that can be used in self-defense.

As fighting intensified across Syria, there were reports that government forces were using helicopters to fire on a rebel-held enclave in the northwestern part of the country. The Syrian Observatory for Human Rights, based in Britain, reported that more than 60 people had been killed in the fighting, one-third of them government soldiers, whilethe United Nations released a report saying that Syrians as young as 8 had been deployed by government soldiers and pro-government militia members as human shields.

The fierce government assaults from the air are partly a response to improved tactics and weaponry among the opposition forces, which have recently received more powerful antitank missiles from Turkey, with the financial support of Saudi Arabia and Qatar, according to members of the Syrian National Council, the main opposition group in exile, and other activists.

The United States, these activists said, was consulted about these weapons transfers. Officials in Washington said the United States did not take part in arms shipments to the rebels, though they recognized that Syria’s neighbors would do so, and that it was important to ensure that weapons did not end up in the hands of Al Qaeda or other terrorist groups.

The increased ferocity of the attacks and the more lethal weapons on both sides threatened to overwhelm diplomatic efforts to resolve the crisis. Kofi Annan, the special envoy of the United Nations and the Arab League, continued to pressure Damascus to halt the violence and to respect a cease-fire. But Mrs. Clinton said that if Mr. Assad did not stop the violence by mid-July, the United Nations would have little choice but to end its observer mission in the country.

Mrs. Clinton, State Department officials said, continues to push for a “managed transition,” under which Mr. Assad would step aside. Russia’s role is viewed as critical, however, and Mrs. Clinton’s claims about helicopter shipments are certain to increase tensions with Moscow less than a week before President Obama is scheduled to meet with President Vladimir V. Putin at a summit meeting in Mexico.

Administration officials declined to give details about the helicopters, saying the information was classified. But Pentagon sources suggested that Mrs. Clinton, in her remarks at a Brookings Institution event, was referring to a Russian-made attack helicopter that Syria already owns but has not yet deployed to crack down on opposition forces. While these helicopters, known as Mi-24s, are flown by Syrian pilots, Russia supplies spare parts and provides maintenance for them.

A Pentagon spokesman, Capt. John Kirby, said the precise status of the helicopters was not as important as the violence being directed against opponents of the Syrian government. “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.” Captain Kirby said. “It’s really about what they’re doing with what they’ve got in their hand.”

The use of helicopters is contributing to a growing sense that, as Hervé Ladsous, the head of United Nations peacekeeping operations, put it, the fighting could be characterized as a civil war.

“The government of Syria lost some large chunks of territories and several cities to the opposition and wants to retake control of these areas,” Mr. Ladsous said at the United Nations. “So now we have confirmed reports not only of the use of tanks and artillery, but also attack helicopters.”

Opposition leaders are wary of the term civil war because it suggests that the conflict is somehow an even match.