BEIRUT — Syrian peace talks in Geneva were at an impasse Friday as Russian and U.S. diplomats exchanged barbs about the deadlock, while in Syria, government troops appeared set for a new offensive against a rebel stronghold.

Negotiators for Russian ally Syria and the U.S.-backed opposition bloc conceded that there had been no progress in the second installment of Geneva peace negotiations, which were frozen in a standoff about the future of Syrian President Bashar Assad.

In Syria, government forces were reported to be massing for an assault against the rebel-held town of Yabroud, in the mountainous Qalamoun area near the Lebanese border. Yabroud has been pounded by airstrikes and shelling in recent days, according to various reports.

The United States said an assault on Yabroud would "undermine the Geneva process and the prospects for peace in Syria," said Edgar Vasquez, a State Department spokesman.

The Syrian military has made significant advances in the rugged Qalamoun area. Yabroud, the last major rebel-held town in the zone, sits a few miles from the strategic north-south highway from Damascus to Homs and the Mediterranean coast, a government bastion.

As government attacks have intensified, refugees from Yabroud have been streaming across the border to the Lebanese town of Arsaal.

There is widespread agreement among diplomats that only a political solution can end the almost three-year-old civil war. The debut session of the Syria peace talks, in January, failed to produce tangible results. But many viewed it as promising that representatives of the warring sides had finally sat down together after months of uncertainty about whether the talks would take place at all.

Few expected a breakthrough in the second round, but the fact that the delegations could not even agree on an agenda was seen as discouraging. Each side accused the other of intransigence. There was little sign of compromise in the delegations' divergent views on the fundamental purpose of the talks.

The opposition asserts that the Geneva negotiations should focus on the formation of a transitional Syrian government that would exclude Assad and his chief allies.

Damascus insists that Assad's future is non-negotiable. Government negotiators have pushed for discussion of how to end "terrorism," as Syrian officials define the armed revolt against Assad.

"We deeply regret that this round did not make any progress," Syria's deputy foreign minister, Faisal Mekdad, told reporters in Geneva.

Separately, Louay Safi, an opposition bloc spokesman, told journalists that the talks had reached a "dead end" because of the government negotiating team's "belligerence."

Given the lack of progress, some expected that the chief U.N. negotiator, Lakhdar Brahimi, would soon adjourn the current round with the hope of scheduling a third negotiating session.

"Where we are right now, I would say, is at a tough place," a senior U.S. official told reporters in Geneva.

The official Syrian media reported Friday that the government and opposition delegations would meet Saturday in Geneva.

Brahimi, known for his perseverance in trying to resolve seemingly intractable conflicts, warned this week that "failure is always staring at us in the face." He had hoped that the presence of high-level U.S. and Russian diplomats in Geneva would help "unblock the situation for us."

Meanwhile, Russian news agencies reported that Russian Foreign Minister Sergei Lavrov accused the opposition and its allies in Washington of undermining the talks by focusing on "regime change." The Geneva discussions, the Russian diplomat said, were "going in circles."

Moscow is a major ally of Assad, and Washington has funneled millions of dollars in aid and equipment to anti-Assad forces. Russia and the United States were the major forces behind organizing the Geneva negotiations.

Lavrov's U.S. counterpart, Secretary of State John F. Kerry, said in Beijing that it would be "revisionist" to suggest that the Syrian talks were about anything other than creating a transitional government in Syria to replace Assad.

On the humanitarian front, Kerry said that the crisis in Syria has "gotten worse, dramatically worse" in recent months, with increasing numbers of casualties, refugees and internally displaced. Kerry labeled the situation "grotesque," putting the blame squarely on the Syrian government.

The Obama administration is pushing for a U.N. resolution that would demand that Syrian authorities allow aid to flow into dozens of besieged communities.

An estimated 240,000 Syrians live in areas blockaded by one side or the other, the U.N. says. Residents in encircled zones lack regular supplies of food and other staples.

Russia is wary of any proposed U.N. resolution, viewing such proposals as a possible pretext to trigger foreign intervention in Syria.

On Friday, Moscow called on Washington and other nations to press Syrian rebels to lift blockades that have denied aid to tens of thousands of civilians trapped in pro-government zones, including the towns of Nubul and Zahra in northern Syria.

patrick.mcdonnell@latimes.com

Special correspondent Nabih Bulos contributed to this report.