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.

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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.

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“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.”

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

“Civil war will not come suddenly in one day or two or five, but you have to look how things are gradually changing on the ground,” said Samir Nachar, a member of the executive committee of the Syrian National Council. “Can you say to people, ‘Don’t defend yourselves?’ It is impossible.”

Council members on Tuesday were also wary of reading too much into Mrs. Clinton’s claim, suggesting that it was an open secret for months that the Russians were supplying weapons to Syria. There have been repeated reports of Russian armament ships docking in Syria, although Moscow has always denied that they were carrying the arms used to suppress the protests.

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Speaking in Istanbul , council members also described efforts to supply the opposition with arms, specifically antitank weaponry delivered by Turkish Army vehicles to the Syrian border, where it was then transferred to smugglers who took it into Syria.

Turkey has repeatedly denied that it is giving anything other than humanitarian aid to the opposition, mostly at refugee camps near the border. It has recently made those camps harder to visit: permission was not granted to two reporters in the vicinity for five days last week. Turkey did not act alone, but with financial support from Qatar and Saudi Arabia and after consultation with the United States, said these officials, speaking on the condition of anonymity because of the subject’s diplomatic delicacy.

The more powerful weapons have been delivered as far south as the suburbs of Damascus, but not into Damascus itself, they said. The presence of the antitank missiles seems to have made government forces hesitant to move their tanks around urban centers, according to sources in the Syrian National Council.

But they have done nothing to stem the violence. On Tuesday, a team of United Nations cease-fire monitors retreated before reaching Al Heffa in the northwest, when hostile crowds struck their vehicles with stones and metal rods, said a spokeswoman, Sausan Ghosheh.

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“The shelling has been continuous,” said Houran al-Hafawi, a member of the local coordination committee of Al Heffa. “The Syrian Army is throwing missiles and rockets from helicopter and rocket launchers from the eastern and western entrances.”

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For the Pentagon, the debate over Russia’s rearming of Syria took an odd twist on Tuesday when Senator John Cornyn , a Texas Republican, complained that the United States military was buying attack helicopters for Afghan security forces from the same Russian weapons company supplying the Assad government.

George Little, the Pentagon press secretary, defended the purchases of the Mi-17 helicopters from the Russian company, Rosoboronexport, as important to helping Afghanistan create a credible self-defense force, and said the issue was separate from the concern over arms shipments to Syria that were used by the government to kill civilians.

“It’s about equipping the Afghan air force with what they need to ensure that they have the capabilities from an air standpoint to defend themselves,” Mr. Little said.