ISTANBUL—The United Nations mission in Syria on Saturday suspended the activities of observers in the country, citing a surge in violence over the past ten days that was impeding the unarmed monitors´ work and putting them in significant danger.

The decision appears to reflect a serious escalation in the armed fighting in Syria, with an April 12 cease-fire agreed to by the government and opposition now in tatters and few diplomatic options to stem the country´s descent into a protracted civil war.

"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," Norwegian Maj. Gen. Robert Mood, head of the U.N. Supervision Mission in Syria, said in a statement.

"This escalation is limiting our ability to observe, verify, report… basically impeding our ability to carry out our mandate," General Mood said. The mission´s suspension ends the work of the monitors who offered some of the only extensive first-hand accounts from the ground at a critical time in the conflict, with mass and gruesome killings on the rise.

U.N. observers will no longer be conducting patrols, Gen. Mood said. The suspension of their activity will be reviewed daily, and the mission aims to "return to normal operations."

The immediate risk the suspension poses is even worse violence and chaos, sixteen months into a conflict in which neither side appears willing to compromise. At least 10,000 people have been killed and thousands more displaced in the Syrian uprising, according to U.N. estimates. President Bashar al-Assad is backed by a retrenching core of loyalists—and, internationally, by Russia and Iran. The regime faces an opposition that has won the backing of Arab Gulf states, Turkey, and the U.S. and its allies, and a ground insurgency that appears to be becoming better-organized and equipped.

Syria´s government said it had been informed on Friday night of the decision to suspend the mission, which it characterized as pointing the blame at armed opposition fighters. "Arab and international parties are still supplying the terrorists with advanced weapons and telecommunications equipment that help the terrorists to commit their crimes and defy the United Nations and its plan," state news agency SANA said.

A surge in violence in January disrupted the work of Arab League monitors in Syria. The end of that mission was followed by a ramped- up military campaign to root out opposition fighters from their strongholds that led government and opposition forces into a spiral of attacks and counterattacks.

"No doubt, the suspension of the monitors will lead to a rising tally of deaths and allow for greater violence from the government," said Abdelbaset Sieda, the newly elected president of the Syrian National Council, the leading opposition coalition-in-exile.

"The time has come to declare the Annan plan a failure," Mr. Sieda said in an interview in Istanbul, where Syrian opposition groups gathered at a meeting with international officials.

"It will now become open warfare," said a young activist who recently fled Syria, declining to be named.

The unraveling of the monitoring mission also raises questions over the future of a broader U.N.-backed peace plan for Syria, and may boost pressure on Russia to help negotiate a political transition amid the urgency of a potential full-blown civil war.

The work of U.N. monitors was one part of a six-point peace plan, brokered by special envoy Kofi Annan, which had won a Security Council mandate with the backing of Russia and China. It represented the broadest international effort on Syria, with Western officials hoping Mr. Assad´s allies on the council could pressure him to stop the crackdown.

Mr. Sieda said the opposition was urging France´s proposal for a Security Council resolution that would outline sanctions against Damascus or authorize the use of force in Syria, action that is still likely to be blocked by Russia and China.

Mr. Annan, meanwhile, is working on bringing major world powers together—now including key Syrian ally Iran—in a meeting to break the diplomatic deadlock. That work continues, officials meeting in Istanbul said, though the U.S. has resisted including Iran in a meeting on Syria.

In Washington, the White House and State Department said the U.S. was consulting with its partners on the next steps in Syria, including a political transition. "The sooner this transition takes place, the greater the chance of averting a lengthy and bloody sectarian civil war," the State Department said in a statement.

In Syria, the violence killed 50 people, according to activist reports, as the government shelled the Damascus suburb of Douma and maintained artillery attacks on the city of Homs. Eleven of those tallied dead on Saturday were bodies found in the Damascus suburb of Saqba that appeared to have been murdered, said the Syrian Observatory for Human Rights, an opposition watchdog based in the U.K.

The Syrian government and opposition have traded blame since April for the apparent failure of the Annan plan. Protesters and opposition fighters say they grew frustrated watching observers they saw as simple bystanders to government attacks and gruesome killings. Regime supporters, too, criticized the mission for appearing to accept a militarized opposition fighting the government´s forces. The result was an environment one U.N. official describe as "extremely, terribly hostile" for the monitors.

The mission´s suspension comes nearly a month before its initial three-month U.N. Security Council mandate comes due. U.N. officials have expressed growing concern over the safety of the unarmed, blue- bereted monitors over the past few weeks, citing near-daily attacks near or on convoys and firing on the monitors. Some 298 military observers are spread across Syria, just a few short of the 300 authorized for deployment.

The mission also has 82 international civilian staff on the ground, working on a range of humanitarian, political, and logistical issues.

—Sam Dagher in Beirut contributed to this article. (Copyright © Dow Jones & Company, Inc.) 06/17/12)