Mr. Annan and others have said the unarmed monitors have already had an effect. But Syrian government forces have attacked cities as soon as monitors have left them, and the opposition has accused the government of assassinating nine activists in Hama who had met with the United Nations team.

How much impact even 300 observers can have is a gamble. Numerous analysts pointed out that the short-lived international observer mission in Kosovo more than a decade ago had 2,000 observers for an area one-seventeenth the size of Syria and with about one-tenth of Syria’s population of 23 million.

In public and privately, senior administration officials made clear that they had no expectation that Syria would implement the Annan plan. Instead, Washington hopes to rely on sanctions; diplomatic pressure; increased engagement with the opposition, including nonlethal aid like bulletproof vests for peaceful activists; and the looming threat of prosecution — all the tools at its disposal short of military intervention.

Stronger action by the United Nations Security Council would require Russian acceptance, but there is no sign that the problems implementing the peace plan are leading in that direction. Although the Russian Foreign Ministry called on Syria last week to “carry out in full its obligations,” it also continued to blame the opposition for the violence. Foreign Minister Sergey V. Lavrov told state television on Friday that the truce had yet to jell “for the most part because armed opposition groups are engaging in provocations, explosions, terrorist attacks and shootings.”

That echoed the attitude in Damascus. The Syrian Foreign Ministry sent a letter to Mr. Annan detailing 1,149 “documented and verifiable violations, by armed elements, of the Annan plan,” said Jihad Makdissi, the ministry spokesman. Security Council members should act “evenhandedly toward violations” and refrain from using the Council as a platform to pursue the policies of individual states, he said without naming any.

Mr. Annan is scheduled to make his next report on the situation byMay 7, and update the Council every 15 days. The United States has said it will not extend the observers’ mandate past the original 90 days if Syria flouts the truce.

A senior administration official, speaking on the condition of anonymity because of the delicacy of diplomatic negotiations, said Washington would not wait the full 90 days to try to end the mission if there was no sign of compliance.