A U.N.-brokered peace plan for Syria appeared close to collapse Sunday as the government demanded a written guarantee that the rebels will lay down their arms before authorities withdraw troops from cities and towns.

The statement cast serious doubt on hopes that the peace plan — the only initiative backed by Syrian allies China, Russia and Iran as well as the United ­Nations, the Arab League and Syria — could quell the violence stemming from a government crackdown on a year-long uprising against the government of President Bashar al-Assad.

Kofi Annan, the joint U.N. and Arab League envoy to Syria, issued a statement Sunday in Geneva, saying he was shocked by “a surge in violence and atrocities” that violated assurances given to him by Syrian officials. Annan had said that the withdrawal of troops from cities was due to be completed by April 10, and that there would be a cessation of hostilities by all sides 48 hours later.

Jihad Makdissi, a spokesman for the Foreign Ministry, said U.N. reports that Syrian officials had said their forces would pull back from cities by the Tuesday deadline resulted from a mis­understanding. He instead presented conditions that were not part of the six-point peace plan hashed out last month by Annan.

Makdissi called for written guarantees from armed groups that they would lay down their weapons in exchange for government forces’ withdrawal from cities, and he demanded that ­Qatar, Saudi Arabia and Turkey withdraw their support for armed opposition, according to Syrian state media.

His statement came after U.N. officials said that they had been informed that Assad had endorsed the plan on April 1 and that his forces had started withdrawing from urban areas, beginning in the troubled Idlib province in the north.

Col. Malik Kurdi, an assistant commander of the Free Syrian Army rebel group, dismissed the government’s new demand. “We do not refuse to give guarantees, but this regime is still shelling and bombarding Syrian cities as well as committing massacres,” he said by telephone from Turkey.

The statements from both sides came after two days of particularly heavy violence in the country, in which activists say 100 to 200 people were killed, including dozens of soldiers. On Sunday, 21 civilians, 12 soldiers and five armed opponents of the government were killed, according to the Britain-based Syrian Observatory for Human Rights. Reports are difficult to verify because Syria restricts journalists’ access to the country.

A spokesman for the Syrian National Council, the most prominent opposition political group, expressed little faith in the Annan plan Sunday. The group previously backed it but said at the time that it feared the regime would use the plan as a delaying tactic to crush the opposition. Instead, spokesman Ausama Monajed said, the group was focusing on unifying a disparate armed opposition movement and encouraging international intervention.

Monajed said it would not be possible for one leader to issue a written guarantee of a cease-fire, as Syrian authorities have demanded, because the armed opposition consists of many groups that operate independently. But he stopped short of saying that there was no hope for the peace plan.

The opposition, backed by some Arab states, including Saudi Arabia and Qatar, has regarded the peace plan as irrelevant and has been more focused on obtaining money and weapons, Fawaz A. Gerges of the London School of Economics said last week. “Even though they disagree about the post-Assad political order, one thing they agree on is militarizing the uprising,” he said.

Should the plan collapse, said Andrew Tabler of the Washington Institute for Near East Policy, the international community should look again at effective ways of supporting an armed opposition, something the West has shied away from.

“Conflict is the constant in Syria for the foreseeable future,” Tabler said. “This is not settling down.”

Some analysts suggested that a failure of the plan would be harmful to Russia, which has been a firm backer of Assad and has supported the peace plan.

“This is ultimately a blow to Russia,” said Richard Gowan, an expert on the United Nations at New York University. “The West supported the Annan plan in the hope that Moscow would force Assad to comply. Assad’s actions this weekend suggest that he holds Russia’s concerns and influence in contempt.”

But Chris Phillips of the University of London said that the Annan plan might still result in some successes, perhaps including the placement of U.N. monitors in the country, and that Russia, Iran and China are unlikely to abandon their ally just yet.

The West, he said, is likely to continue to resist supplying arms to an opposition that is fragmented and that could attract elements of regional terrorist groups if it becomes a full-fledged insurgency.

“The West is trying to tread a delicate line between encouraging the opposition, encouraging regime change and not turning Syria into a failed state,” he said.

Staff writer Colum Lynch at the United Nations contributed to this report.