The White House issued a statement on Saturday once again calling on Syria to uphold commitments it has made in recent months, “including the full implementation of a cease-fire.” The statement added, “We are consulting with our international partners regarding next steps toward a Syrian-led political transition” called for in two United Nations Security Council resolutions, and “the sooner this transition takes place, the greater the chance of averting a lengthy and bloody civil war.”

Syria’s uprising has become one of the most intractable and deadliest conflicts of the Arab Spring, with reports of at least four massacres in recent weeks, including accounts of killings of as many as 78 civilians, many of them women and children.

On Saturday, dozens of Syrians were killed in government attacks across the country, especially in villages around Damascus, the capital, according to the Syrian Observatory for Human Rights, a group based in Britain with contacts in Syria. The group and other activists said security forces were carrying out sweeping arrests, particularly of young men, in towns around the capital.

The Obama administration is resisting calls to arm rebel groups, for fear that they are not an organized force and could eventually turn on one another. “The problem is that if we do nothing and Syria explodes, we have a broader conflict in the Middle East ,” a senior American diplomat said last week, before the United Nations announcement, adding, “But our options aren’t any better than they were a year ago.”

The observers had been the foundation of a six-point peace plan that Kofi Annan , the former United Nations secretary general and the special envoy to Syria, had sought to hammer out with the consent of Mr. Assad and his foreign sponsors, including Russia and Iran .

Both of those countries have huge stakes in the outcome: Russia has a military base in Syria and has long used Mr. Assad as an instrument to project influence in the region, and the Syrian government is Iran’s only real ally in the region. But Russia has frozen strong action, complaining that the West went beyond its humanitarian mandate when it aided the overthrow of Col. Muammar el-Qaddafi in Libya last year.

The leader of the observer mission in Syria, Gen. Robert Mood, said in a statement that he had little choice but to suspend the mission. Escalating violence across Syria over the past 10 days had prevented the teams from carrying out their mandate to verify events on the ground. They have repeatedly been attacked by pro-government supporters, driving them back in recent days from the village of Al Heffa, which had been under assault all week until all its residents fled.

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“The lack of willingness by the parties to seek a peaceful transition, and the push toward advancing military positions is increasing the losses on both sides: innocent civilians, men, women and children are being killed every day,” General Mood said. “It is also posing significant risks to our observers.” But he emphasized that he was only suspending the mission, not ending it, and would evaluate daily the chances for resuming its activities.

Ahmad Fawzi, Mr. Annan’s spokesman, said General Mood was responding in part to pressure from countries that contributed the observers.

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“Troop-contributing countries are saying our men and women are at risk, we are having second thoughts about this operation,” Mr. Fawzi said. “They are in danger and they want the danger to go away.”

“There is nothing final,” he said. “It is a suspension, not termination.” But patrols would resume only “when we return to a situation where both sides show us that they are serious and earnest about stopping the killing of each other.”

General Mood is expected to fly to New York to brief the Security Council on Monday.

Responding to the observers’ decision, Syria’s government said Saturday that it respected both the peace plan and the safety of the United Nations observers. But it blamed the opposition for the escalation of violence in Syria. In a statement, the Foreign Ministry also assailed “Arab and international powers” for arming the rebels and supporting their “defiance of the U.N. plan.”

By mid-July the original 90-day mandate for the observers will expire. But the suspension, if prolonged, will focus new pressure on those governments allied with Syria, particularly Russia and Iran, which have backed the plan as the only way to stop the violence.

The inclusion of Iran in an international group proposed by Mr. Annan to discuss ways to save the peace plan — a “contact group” in the United Nations’ parlance — was rejected by the United States , while Russia insisted on it. That raised serious questions about whether the contact group would ever meet.

At a minimum it is supposed to include the five permanent members of the Security Council, plus important neighboring states. “Everybody realizes this meeting has to take place,” Mr. Fawzi said. “They need to come up with a draft action plan.”

That plan is supposed to include a cease-fire and a political transition, presumably with Mr. Assad leaving the country.

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But Russia and Iran have continued to back Mr. Assad and have refused to endorse plans for his exit. The United States, along with Saudi Arabia and other Arab states, has embraced the rebels. The inability of the United Nations to bridge the gap only increases the likelihood that the Syrian conflict will become a regional proxy fight.

“There is just a political hurricane gathering in the Eastern Mediterranean,” said Andrew J. Tabler, a Syria scholar at the Washington Institute for Near East Policy. “Both sides agree that it is a hurricane, but they don’t agree on its nature.

“And in any case,” he continued, “I am not sure how you could contain this. Even if you started now, it would take a long time to get something in place that would tamp this down.”

Inside Syria, opposition activists called the observer mission a sham that had only served to deflect attention from the failure of the world powers to stop Mr. Assad’s forces from killing civilians.

“Their presence is just like their absence,” Mohammed el-Muetassem bi’Allah, 18, an activist from Homs, said of the observers. “They are incapable of stopping the violence. They were there and the shelling was intensifying on Homs and Khaldiya.”