Syrian forces killed dozens of civilians in a village in central Syria, opposition groups and witnesses said Saturday, amid growing questions about the effectiveness of a U.N. monitoring mission that is supposed to be observing a cease-fire. U.N. officials said that at least 32 children were among the dead.

More than 90 people were killed and many more wounded when Syrian troops loyal to the government of President Bashar al-Assad shelled the village of Taldo in the Houla area northwest of Homs for several hours late Friday, according to the opposition groups and witnesses.

It appeared to be one of the bloodiest single incidents of the 14-month-old uprising against Assad’s rule and the deadliest since a fragile, U.N.-brokered cease-fire went into effect April 12.

The violence drew harsh international condemnation. A statement issued by U.N. Secretary-General Ban Ki-moon and the joint U.N.-Arab League special envoy Kofi Annan said U.N. monitors had visited the area and had confirmed activist reports that dozens of civilians were killed in the shelling of the village.

“This appalling and brutal crime involving indiscriminate and disproportionate use of force is a flagrant violation of international law and of the commitments of the Syrian Government to cease the use of heavy weapons in population centers and violence in all its forms,” the statement said. “Those responsible for perpetrating this crime must be held to account.”

Secretary of State Hillary Rodham Clinton said that the United States would work “with the international community to intensify our pressure on Assad and his cronies, whose rule by murder and fear must come to an end.”

The head of the U.N. mission, Maj. Gen. Robert Mood, said that the dead included at least 32 children younger than 10 and that U.N. observers found artillery tank shells at the scene, the Associated Press reported.

The Britain-based Syrian Observatory for Human Rights and the Syrian Muslim Brotherhood offered conflicting accounts of what happened. The Observatory said that all of the deaths had occurred in the bombardment, but the Brotherhood reported that some of the victims had been killed when pro-government militias known as shabiha raided homes on the outskirts of the village, hacking and shooting civilians and setting fire to houses.

Ahmed Kassem, a resident of the town, said the villagers all died in the shelling inflicted after clashes erupted during the weekly anti-government protest Friday.

Syrian forces opened fire on the protesters when they spilled out of a mosque after Friday prayers, prompting local armed civilians to fire back, Kassem said, speaking by telephone from the village.

During the exchanges of fire, two Syrian officers and “several” soldiers were killed, he said, and Syrian forces withdrew. At 8 p.m., they began bombarding the village using tanks and artillery, with shells falling at the rate of one a minute until well past midnight, he said.

The account could not be independently confirmed because of reporting restrictions imposed by the Syrian authorities. Activists posted gruesome footage on YouTube of the limp bodies of children and of a mass burial of some of the victims Saturday.

The official government news agency SANA did not mention the bombardment. But it reported what it described as “two horrible massacres against a number of families” in Taldo and blamed “Al Qaeda-linked terrorist groups.” The agency posted blurred pictures of what it said were the victims of the killings.

The bloodshed further called into question the efficacy of the already fragile cease-fire, and of the month-old U.N. mission to monitor it. There are currently 271 monitors in Syria out of an eventual total of 300, according to the United Nations.

In a grim assessment of the situation delivered in a letter Friday to the U.N. Security Council, Ban acknowledged that there had been little progress toward the implementation of a six-point, U.N.-mandated peace plan aimed at halting the violence and initiating a process of political reform.

“The overall situation in Syria remains extremely serious,” the letter said. “There is a continuing crisis on the ground, characterized by regular violence, deteriorating humanitarian conditions, human rights violations and continuing political confrontation.”

Syrian troops and tanks are still deployed in residential areas in violation of the terms of the plan, and the many thousands of political detainees believed to be imprisoned have not been released, the letter said.

It also noted “an alarming number of explosions in population centers, including acts of terrorism.”

Moreover, it added, “significant parts of some cities appear to be under the de facto control of opposition elements,” an indication that the government’s crackdown is not succeeding in crushing the revolt.

“There is an overall atmosphere of tension, mistrust and fear,” the letter said.