Syrian forces resumed their shelling of a rebel stronghold in the city of Homs on Friday, a day after world leaders sought to apply new pressure on the government of President Bashar al-Assad.

Activists and human rights groups said the neighborhood of Khaldiyeh in the central Syrian city came under artillery fire Friday morning.

With Syria lurching closer to sectarian warfare, world leaders on Thursday said the Assad government showed no sign of honoring cease-fire agreements or halting the slaughter of noncombatants, but they remained divided on how to revive a peace process that U.N. officials conceded is in ruins.

As new details emerged of the latest massacre of civilians, U.N. officials spoke for the first time of unspecified “consequences” for the Syrian government. Yet the refusal by Russia and China to embrace new economic sanctions meant that the world body was left without a clear path forward.

“Pressure — substantial pressure — is what this crisis demands,” U.N. special envoy Kofi Annan said during a closed meeting of the U.N. Security Council, according to a diplomat who attended the session. “We will soon reach a day when it will be impossible to keep the crisis from spiraling out of control.”

The warnings came less than 24 hours after news broke of the second mass killing of Sunni civilians in as many weeks. Up to 78 people, most of them women and children, were reported slaughtered in Qubair, a small village in Hama province, by forces loyal to Assad.

Syrian troops blocked U.N. monitors from entering the village, and some observers came under small-arms fire as they tried to approach the town, U.N. Secretary General Ban Ki-moon said. He provided no details about who had fired at the monitors or whether there were any injuries. U.N. observers have frequently been shot at since their arrival in Syria in April to monitor a fragile cease-fire.

Ban expressed revulsion at the “shocking and sickening” reports of massacres and called on the Syrian government to immediately implement the U.N.-backed plan for a cease-fire and political transition. But later, Ban and Annan acknowledged that the peace plan was approaching a dead end, with both sides refusing to adhere to it.

Ban presented the U.N. General Assembly with a gloomy assessment of conditions in Syria, asserting that the “dangers of full-scale civil war are imminent and real.” He held Assad primarily responsible for the worsening violence.

“For many months, it has been evident that President Assad and his government have lost all legitimacy,” Ban said. The slaughter of civilians in the village of Houla two weeks ago “brought this fact into horrifying focus,” he said. “Men, women, even children were executed at point-blank range, some of their throats slit or skulls crushed.”

Of the Qubair incident, he said the village had been “apparently surrounded by Syrian forces . . . the bodies of innocent civilians lying there . . . they were shot . . . some allegedly burned or slashed with knives,” Ban said. “We condemn this unspeakable barbarity.”

Ban and Annan pleaded for a reinvigorated diplomatic push to avert a full-fledged civil war and warned, for the first time, that any party blocking a political transition should face “consequences.”

“Clearly, the time has come to determine what more can be done to secure implementation of the [six-point peace] plan — and what other options exist to address the crisis,” Annan, a former U.N. secretary general, told the General Assembly at a special session on Syria. “If things do not change, the future is likely to be one of brutal repression, massacres, sectarian violence and even all-out civil war. All Syrians will lose.”

Terror in Sunni village

It was not possible to independently confirm many details of the massacre Wednesday afternoon in Qubair, a Sunni hamlet northwest of the city of Hama that is surrounded by villages populated by members of Assad’s Shiite-affiliated Alawite sect.

But activists and residents contacted in the area described circumstances similar to those in Houla, a Sunni village northwest of Homs where pro-government militiamen — known as the shabiha — massacred 108 civilians May 25, allegedly with the aid of security forces.

Qubair resident Laith Hamawi said his mother and brothers were among the victims. He said he was about half a mile away in his olive fields when he saw security forces and shabiha members converge on the village of about 150 residents from three directions.

“I was scared to move, so I hid,” he said in an interview.

For several hours, Hamawi said, he heard shooting and tank fire and saw houses burning. After the troops left, he said, he returned home and found the bodies of his mother and brothers.

Most of those killed in Qubair were from the Yateem family, and 40 were women and children, other activists said. They said that government forces took at least 30 of the corpses with them and that other bodies were burned when homes were set on fire. Residents waited much of the day to show the remaining corpses to U.N. monitors, but when they did not arrive, the villagers buried the dead.

The official Syrian Arab News Agency denied as “absolutely baseless” the reports that Assad loyalists had carried out the killings. Government forces moved into the area and killed “terrorists” after an “armed terrorist group” killed nine women and children, the news agency said.

No consensus on response

Annan has been seeking support for a plan to set up a new negotiating bloc — or contact group — that includes representatives from the United States, Russia, Iran, and other regional and global powers, according to diplomats familiar with the proposal. The group would be tasked with drawing up a transition plan, including presidential and parliamentary elections, and using its influence to persuade the rival Syrian parties to accept the proposal.

Despite Annan’s call for unity, sharp differences remain between, on the one hand, Western and Arab governments, which favor the imposition of sanctions on Syria, and, on the other, China and Russia, which have resisted such measures and oppose any international effort to oust the Syrian leadership.

In a possible sign of softening in Russia’s position, Vitaly Churkin, Moscow’s ambassador to the United Nations, told his Security Council counterparts that Russia could accept a political transition in Syria that resulted in Assad stepping down.

“We are not wedded to Assad. We have had that position from the start,” Churkin said, according to a Security Council official present during the closed session. “If he had to go, as a result of a political process, we wouldn’t be so upset.”

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