A team of U.N. monitors began on Friday the grim task of investigating a massacre in central Syria, as new evidence of horrific violence gave momentum to calls for international sanctions to halt the bloodshed.

The visit to the village of Qubair, where at least 78 people were reported killed by pro-government militias this week, came a day after the monitors had been blocked by Syrian security forces from reaching the site.

The U.N. personnel, accompanied by a handful of reporters, encountered a virtual ghost town inhabited by swarms of flies and reeking of charred flesh, according to U.N. officials. Video footage released by the United Nations showed a series of disturbing scenes, including one of blood-spattered walls and another of homes bearing the pockmarks that have become the signature of the Syrian government’s shelling campaigns.

U.N. spokesman Kieran Dwyer said that the monitors were unable to talk to any witnesses of the attack but that they found evidence of fresh armored-vehicle tracks and homes damaged by rocket fire, grenades and a range of other weapons fire.

The “circumstances surrounding this attack are still unclear,” Dwyer said.

Surviving residents of the tiny village, a cluster of about 20 homes, said security forces had visited them the night before and threatened them with death if they cooperated with the monitors. Nonetheless, one resident said he covered his face and led the monitors on a tour of the devastation.

“We took them to the graves where we buried the bodies, we showed them the burned houses and the bloodstains in the other houses,” he said, speaking on the condition of anonymity because he feared for his safety.

The evidence of what would be the second large-scale killing of Syrian civilians in three weeks has put additional pressure on the United States and its allies to scramble for a new strategy to contain the deepening sectarian violence. On Friday, in another indicator of the escalating violence, unusually fierce clashes broke out in Damascus, with residents reporting gunfire and explosions late into the night. And early Saturday, Syrian troops shelled the southern city of Daraa, killing at least 15 people, activist groups said.

Diplomats at the United Nations said representatives of the United States, Britain and France planned to begin work next week on a legally binding resolution that would for the first time impose international sanctions on Syria if it fails to halt its crackdown on civilians.

The push to ratchet up pressure reflects deepening concern that the prospects for averting a full-fledged civil war — and possibly regional unrest — are slipping away.

U.S. officials, in particular, are pushing Russia, which has withheld support of international action in Syria, to abandon President Bashar al-Assad, tighten bilateral sanctions, and support efforts by the armed and political opposition to challenge Assad’s rule.

In Washington, after meeting with Secretary of State Hillary Rodham Clinton, U.N. special envoy Kofi Annan acknowledged that his peace plan was not working and that curbing the violence in Syria was “a real, real challenge.” He said that he and Clinton had discussed “how we can put additional pressure on the government, on the parties to get the plan implemented.”

“Some say the plan may be dead. Is the problem the plan, or the problem is implementation?” Annan said as he stood next to Clinton. “If it’s implementation, how do we get action on that? And if it’s the plan, what other options do we have?”

State Department officials said Annan’s discussions with Clinton focused on getting countries to line up behind plans to both increase pressure on the Syrian government and create a blueprint for a peaceful transfer of power after Assad is gone.

Annan has met separately with Russian officials, and on a Friday, a special U.S. envoy held parallel meetings in Moscow, amid what U.S. officials termed a “constructive” effort to narrow differences.

Despite Russian resistance to adding new economic pressure against its ally, separate teams of diplomats were continuing to explore the possibility of new sanctions, “to both tighten them and look at how we can pile more on,” unilaterally if necessary, said State Department spokeswoman Victoria Nuland.

She suggested that that a tougher U.N. resolution could be considered “when the moment is appropriate” but declined to elaborate. Both Russia and China have opposed any consideration of resolutions authorizing mandatory sanctions or military force against Syria.

Representatives from Britain and France said that they would begin pressing for the adoption of a legally binding resolution and that the United States would back that effort.

Diplomats said that no one has formally produced a draft negotiating text but that Britain and France are weighing several possible measures, including an arms embargo, a travel ban and asset freeze on key regime figures, the establishment of a commission of inquiry to probe crimes, and a referral to the International Criminal Court for possible prosecution of regime leaders.

It remained unclear whether all of those elements would be included in the draft, or whether the United States would support a referral to the criminal court, which it has never joined.

Annan, meanwhile, has floated a proposal to establish a new negotiating bloc — or contact group — that would include the United States, Russia, Iran, Turkey, Saudi Arabia and other key U.N. and regional powers.

Obama administration officials remained cool to the idea of including Iran in international talks. Clinton dismissed the idea in a news conference Thursday, suggesting that Iran’s hands were too bloody.

“It’s extremely hard to imagine that a country that has played such an extreme role in supporting and perpetrating and — supporting the violence that’s going on could be constructive in this context,” Nuland said. She said Iran had provided support not only for Syria’s security forces but also for the pro-Assad “shabiha” militants accused of carrying out the recent massacres of civilians.

But Russia has warmly embraced the plan and has called for convening a meeting of the new group in Moscow as soon as possible.

Joshua Landis, director of the Center for Middle East Studies at the University of Oklahoma, said any plan for a negotiated solution is unlikely to succeed.

“There is no soft landing for this regime,” Landis said. “Everything is built on loyalty to the Assad family. Once the Assads step down, this entire edifice of this regime is going to disintegrate into a giant cloud of dust.”

Sly reported from Beirut. Special correspondent Suzan Haidamous in Beirut and staff writer Joby Warrick in Washington contributed to this report.