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.

“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.

“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.”