The UN observer mission to Syria has suspended its activities, saying escalating violence is impeding the monitors' ability to carry out its mandate.
"UN observers will not be conducting patrols and will stay in their locations until further notice," the mission's chief General Major Robert Mood said in a statement on Saturday.
About 300 observers are deployed in Syria, tasked with monitoring a ceasefire and supporting the full implementation of a six-point peace plan drafted by UN-Arab League envoy Kofi Annan, which was supposed to lead to talks between the two sides.
However, hundreds of people have been killed since the first observers were deployed in April and the mission has been harshly criticised by the opposition.
Syria's foreign ministry said Mood had informed the government of the decision to temporarily reduce the work of the monitors, Syria's official SANA news agency reported on Saturday.
"The ministry understands the decisions taken by General Robert Mood, particularly those related to preserving the safety of observers," SANA's report said.
Mood said the suspension would be reviewed on a daily basis and operations would resume when the situation was fit.
"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," his statement said. "It is also posing significant risks to our observers."
He said intensifying violence in the past 10 days was "limiting our ability to observe, verify, report as well as assist in local dialogue and stability projects".
Al Jazeera's Anita McNaught, reporting from Istanbul, where Syrian opposition groups were meeting on Saturday, said the reaction to the move by the UN mission was "a lack of surprise on the whole".
"One delegate said to me [the monitors] were there to observe and, from where they sat, the regime had done everything to obstruct the mission to go in and actually view things," she said.
In-depth coverage of escalating violence across Syria
"So in a sense they weren't being able to do the full job they were sent in to do so what was the point of them being there anymore?"
Another opposition member told our correspondent he felt the mission's "days were numbered" after an attack last week on a UN convoy trying to reach the town of Haffeh.
Shots were fired at the car carrying the observers after they were turned away from Haffeh by angry supporters of President Bashar al-Assad, who threw stones and metal rods at their convoy, a spokeswoman for the monitors said at the time.
Saturday's suspension signals a major setback for Annan's plan as the conflict that began in March 2011 with peaceful protests challenging the regime spirals closer to full-blown civil war.
Activists say about 14,000 people have been killed in the conflict.
The White House said it was consulting international partners on the "next steps".
"We call again on the Syrian regime to uphold its commitments under the Annan plan, including the full implementation of a ceasefire," White House spokesman Tommy Vietor said in a statement on Saturday.
"At this critical juncture, we are consulting with our international partners regarding next steps toward a Syrian-led political transition as called for in Security Council resolutions," he said, but provided no specifics.
Western powers have pinned their hopes on the Annan plan, in part because there are no other options on the table.
The international community has little appetite for the military intervention that helped topple Libya's Muammar Gaddafi, and several rounds of sanctions have failed to stop the bloodshed.
But McNaught said that some Syrian opposition members felt the suspension of activities may increase pressure on Syria, as well as on Russia and China, "who have been so staunchly behind the Annan plan and the implementation of it in any shape or form".
"This might increase the pressure on them to do something else constuctive to bring some kind of a resolution or de-escalation of the situation in Syria," she said.
The suspension came as Syrian opposition bloc the Syrian National Council (SNC) warned on Saturday of a looming massacre in the central city of Homs, which it said was besieged by 30,000 troops and pro-government armed groups.
"Regime forces are escalating their shelling of the city of Homs in an unprecedented way," said the exiled opposition bloc, adding troops were "getting ready to carry out a savage attack that might massacre the city's remaining residents".
Activists said shelling and gunfire killed five people in Homs on Saturday. The Syrian Observatory for Human Rights said another 22 civilians were killed elsewhere in the country.
The SNC has asked the UN Security Council to "protect Homs and other areas of Syria that are targeted" by regime forces.