BEIRUT — U.N. observers suspended their patrols in Syria on Saturday due to a recent spike in violence, the strongest sign yet that an international peace plan was unraveling despite months of diplomatic efforts to prevent the country from plunging into civil war.

The U.N. observers have been the only working part of a peace plan brokered by international envoy Kofi Annan, which the international community sees as its only hope to stop the bloodshed.

The plan called for the foreign monitors to check compliance with a cease-fire that was supposed to go into effect April 12, but they have become the most independent witnesses to the carnage on both sides as government and rebel forces have largely ignored the truce.

Maj. Gen. Robert Mood, the U.N. mission chief, said intensifying clashes over the past 10 days were “posing significant risks” to the 300 unarmed observers spread out across the country, and impeding their ability to carry out their mandate.

The observers will not leave the country but will remain in place and cease patrols, Mood said in a taped statement, adding that the suspension would be reviewed on a daily basis.

Teams have been stationed in some of Syria’s most dangerous cities, including Homs and Hama.

“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,” Mood said.

The decision came after weeks of escalating attacks, including reports of several mass killings that have left dozens dead.

The United States reiterated its call for the Assad regime to comply with the plan, “including the full implementation of a cease-fire.”

Underscoring the dangers, activists reported at least 50 people killed in clashes and shelling in several Syrian cities.

The peace plan’s near-collapse has increased pressure on the international community, including President Bashar Assad’s staunch allies Russia and China, to find another solution. But there has been little appetite for the type of military intervention that helped oust Libya’s Moammar Gadhafi, and several rounds of sanctions have failed to stop the bloodshed.

Najib Ghadbian of the main Syrian opposition group, the Syrian National Council, said the concerns expressed by the U.N. mission could pressure Russia to allow more censure of Assad’s regime.

Despite fears that violence could significantly worsen without U.N. monitors on the ground, activist Rami Abdul-Rahman of the Britain-based Syrian Observatory for Human Rights said their numbers were too small, and the conflict too large, for them to have any use.