Activists reported fighting across Syria on Tuesday, the day that U.N. officials, world leaders and Syrian protesters had hoped Syrian forces would complete a pullback from urban areas, while Russia, one of President Bashar al-Assad’s principal allies, indicated it was open to changing the conditions of a United Nations-backed peace plan.

At the end of a 10-day period that Syrian officials had pledged to use to carry out a withdrawal, according to U.N.-Arab League envoy Kofi Annan, Syrian Foreign Minister Walid al-Moualem met with officials in Moscow and voiced further demands.

Saying that government forces had begun to pull back from unspecified areas, Moualem sought U.N. guarantees of an opposition cease-fire as a precondition to the rest of the six-point plan.

“We would like Mr. Annan to deliver us these guarantees and to inform us of his contacts with the opposition,” Moualem told journalists in Moscow. He said the Syrian government was prepared to enact a cease-fire to coincide with the arrival of an international group of monitors from countries of Syria’s choosing.

Annan, speaking in Turkey, rejected any preconditions for halting the violence. He urged Syria to fulfill the terms of his truce plan, which, in addition to the troop pullback, includes the cessation of hostilities by all sides by 6 a.m. Thursday.

In a letter sent Tuesday to the U.N. Security Council, Annan expressed deep concern about the violence. “The prevailing security and human rights situation is unacceptable,” he said, adding that the average number of Syrian refugees crossing into Turkey each week has increased in the past 10 days from 96 to 707.

In his letter, Annan noted that government forces have withdrawn from some centers of fighting, including Idlib, Zabadani, Deir al-Zour and Daraa, but he also described “rolling military operations” that he said included heavy artillery fire on civilians.

The Syrian government is battling a year-old uprising, with increasingly violent clashes taking place between the army and the rebels. International groups and Western leaders have condemned the brutality of the Syrian security forces and called for Assad to step down.

Although Annan’s letter to the Security Council sounded a despondent note about the peace plan, the former U.N. chief advocated continuing efforts to implement it. He said that all opposition groups had assured him they would end hostilities on Thursday, provided security forces had withdrawn from cities.

Russia, whose support for the deal is considered crucial, appeared to be backing away from some of its key requirements, with Foreign Minister Sergei Lavrov saying that Syria needed only to begin a troop pullback by Tuesday, rather than completing it, to fulfill its commitment.

Lavrov said the Syrian government should live up to its side of the deal “more actively,” according to the Interfax news service. But he focused much of his criticism on Syria’s opposition, saying that its fighters, too, needed to cease violence.

Thirty-eight civilians were killed Tuesday, according to the Syrian Observatory for Human Rights, some in Homs and some north of Hama. As many as 19 soldiers were killed close to the Turkish border and in the eastern Hassakah region in several attacks on checkpoints, and five armed opponents of the government were killed across the country. It was not possible to corroborate those reports because Syria restricts journalists’ access.

The continuing violence, which spilled over into Lebanon and Turkey on Monday, has provoked increasingly furious responses from foreign officials.

Sens. John McCain (R-Ariz.) and Joseph I. Lieberman (I-Conn.) released a joint statement Tuesday during a trip to Turkey that they say has included meetings with Turkish leaders, refugees and armed opponents of Assad. The two called for “a concerted international effort to provide these brave fighters in Syria with the capabilities to defend themselves” and for safe havens in Syria defended by international forces, including air power.

Also Tuesday, British Foreign Secretary William Hague accused Assad’s government of using the deadline “as a cover for intensified military efforts to crush Syria’s opposition.” Hague added that if Annan’s peace plan failed entirely, Britain would be ready to go to the Security Council to seek a “united international response to this clear threat to international peace and security.”

Birnbaum reported from Moscow.