The Obama administration on Tuesday accused Russia of planning to supply new attack helicopters to Syria, a move that U.S. officials warned would dramatically escalate the crisis and belie Moscow’s claim that it is not supporting President Bashar al-Assad’s violent crackdown on dissent.

Russia, a longtime ally and supplier of arms to the Syrian government, has acknowledged that it is continuing to honor contractual military shipments to Damascus, while stressing that it is not providing weapons that could be used against opponents of the government. But in an unusually direct rebuke, Secretary of State Hillary Rodham Clinton on Tuesday dismissed such assertions as “patently untrue.”

“We have confronted the Russians about stopping their continued arms shipments to Syria,” she said.

Russia’s shipments, while legal, set the stage for a new diplomatic confrontation with the Obama administration at a time when Washington and its European and Arab allies have been struggling to persuade Moscow to step up diplomatic pressure on Assad to bring an end to nearly 15 months of bloodshed. The shipments — which U.S. officials said were thought to be en route to Damascus — would arrive as the Syrian government escalates its use of force against opposition strongholds, including the deployment of ­Russian-built attack helicopters from its existing reserves.

Underlining the growing concerns, the United Nations’ top peacekeeping official on Tuesday acknowledged for the first time that Syria is in the midst of a civil war.

“Clearly what is happening is that the government of Syria lost some large chunks of territory, several cities, to the opposition and wants to retake control,” the official, Herve Ladsous, told the Reuters and Agence France-Presse news agencies in an interview.

Asked whether the conflict could be characterized as a “full-scale civil war,” he answered: “Yes, I think we can say that.”

Escalating dangers

U.S. and U.N. officials have said that the violence in Syria is increasingly taking on the dimensions of a sectarian conflict, with ­government-backed Shiite militiamen from the minority Alawite sect battling anti-Assad forces drawn from the Sunni majority.

The violence is also increasingly threatening the 300 or so U.N. monitors in the country. On Tuesday, a team of observers trying to reach a northern town engulfed by fighting for the past week were turned back by angry crowds throwing stones and metal rods. Three of the team’s vehicles were fired upon, but the United Nations later said in a statement that the source of the gunfire was unclear.

The observers had been trying to reach the town, Haffah, since Thursday, three days after government troops launched a major offensive aimed at driving out fighters of the rebel Free Syrian Army. The town lies on the edge of the heartland of Assad’s Alawite sect and is surrounded by Alawite villages, raising fears that Haffah’s residents could be massacred if government troops succeed in crushing the rebel fighters.

Dozens are dying daily in attacks on protesters and in the shelling of towns and cities across Syria as the government battles to assert control over the many areas that have fallen under the sway of the rebels.

On Tuesday, 51 people were killed nationwide, including 10 who died in the eastern town of Deir al-Zour when Syrian forces fired mortar rounds at an anti-government demonstration and five members of a family who died when a mortar shell landed on their home, according to the Britain-based Syrian Observatory for Human Rights. The U.N. monitoring mission said observers witnessed heavy shelling over the city of Homs on Monday, as well as devastation in nearby towns that the government is trying to retake.

Ahmad Fawzi, a U.N. spokesman, told reporters in Geneva that the mission is amassing evidence about the government’s use of attack helicopters.

“If they had been used before, it has not been documented; this time, it has been documented,” Fawzi said. “Our observers have videotaped helicopters in the skies with fire coming out of them . . . so whether [they are] helicopters with machine guns on them or helicopter gunships . . . we have not been able to make that distinction yet, but, yes, they are being used, and we have observed them being used.”

Arms caches increasing

Clinton, speaking at the Brookings Institution, said officials are extremely concerned about the possibility of the Syrian government acquiring more attack helicopters. She acknowledged that Syria is increasingly awash in arms, supplied not only by Russia but also by Arab countries backing the rebels.

“There are lots of weapons on the black market,” she said. “There’s money that’s available. There seems to be an increasing capacity in the opposition both to defend themselves and to take the fight to the Syrian military in an irregular way. But there’s no doubt that the onslaught continues, the use of heavy artillery and the like.”

Russian President Vladimir Putin has defended the arms sales to Syria, telling reporters at a news conference Friday in Berlin with German Chancellor Angela Merkel that “Russia is not supplying arms that could be used in civil conflicts.”

Igor Sevastyanov, deputy CEO of the Russian state-controlled arms company Rosoboronexport, reaffirmed the government’s position during remarks Tuesday in Paris.

“No one can ever accuse Russia of violating the rules of armaments trade set by the international community,” said Sevastyanov, according to Russia’s RIA Novosti news agency.

Correspondent Will Englund in Moscow and staff writer Joby Warrick in Washington contributed to this report.