CAIRO — The United Nations said Saturday that it was suspending its observer mission in Syria because of the escalating violence, the most severe blow yet to a wobbly United Nations-negotiated peace plan that the United States, Russia and other world powers had embraced as the best chance to resolve the deadly conflict between President Bashar al-Assad and the rebels challenging his rule.
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.
The suspension of the observers’ mission was the clearest sign yet of the failure of Mr. Annan’s plan to find traction and of Syria’s steady descent toward a bloody civil war.
Gen. Robert Mood, the leader of the United Nations observer mission in Syria, said in a statement that the escalating violence across Syria over the past 10 days had prevented the teams from carrying out their mandate to verify events on the ground.
“The lack of willingness by the parties to seek a peaceful transition, and the push towards 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.”
He said that the mission was only suspended, not canceled, and that the observers would continue to evaluate the possibility of a resumption of their activities.
With the original 90-day mandate for the observers set to expire by mid-July, its early demise will focus new pressure on those governments allied with Syria, particular Russia and Iran, who have backed the plan as the only way to stop the violence.
For President Obama, the suspension of the observers’ activities appears to signal the failure of the latest, and perhaps the last, effort by the West to reach a diplomatic solution that would ease Mr. Assad from power. Mr. Obama is now faced with a vexing choice. A bombing campaign like the one conducted by NATO in Libya last year is not feasible in Syria: the battle is being waged in crowded cities, with little chance to attack the Syrian Army without the risk of high civilian causalities. Yet there is no enthusiasm among NATO nations or the Arab League to send in a ground force and become part of what many fear is emerging as a civil war.
Most American officials are still strongly opposed to arming the disparate rebel groups for fear that they are not an organized force and that the groups 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 early last week, before the United Nations announcement, adding, “But our options aren’t any better than they were a year ago.”
The inclusion of Iran in an international group proposed by Mr. Annan to discuss ways to save the peace plan had been rejected by the United States, while Russia insisted on it. That raised serious questions about whether the contact group would ever meet.
The suspension of the observers’ operations came a day after General Mood, speaking at a news conference in Damascus, said that countries that were contributing troops to the observer mission had expressed concerns about the safety of the unarmed monitors. They have repeatedly been attacked by pro-government supporters, the ferocity of the attacks driving them back from the village of Al Heffa, which had been under assault all week until all its residents fled.
The observers’ decision was in many ways a reflection of the deadlock over the question of Syria’s future among international powers.
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 who are animated by the other Arab Spring uprisings and are demanding Mr. Assad’s ouster. 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, an 18-year-old 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.”
An activist using the name Abou Rawan, 28, a member of the armed opposition’s Free Syrian Army in Rastan, said it had long ago told the observers to leave for their safety. “The observers became targets as well recently because they were transmitting the same image that we were transmitting to the world,” he said.
Dr. Haytham Manna, president of National Coordination Body for Democratic Change in Syria abroad, said the observer mission’s suspension was a loss nonetheless.
“If the monitors leave do you think escalation of violence will decrease?” he said in a telephone interview from Paris. “Violence is violence.”
He said the pullback of the observers would have a psychological effect, increasing Syrians’ fear of leaving their homes and eliminating a vital conduit of information to the outside world.
“Who will be the witness?” he asked. “People will be deprived.”
An activist in Damascus, speaking on the condition of anonymity for reasons of safety, said the absence of the observers would deter protesters who had been emboldened by their “symbolic presence.” But this activist said the observers’ decision would hardly change the pace of violence.
“Violence never stopped in Syria,” the activist said. “Actually, it escalated with the presence of the U.N. monitors.”