UN observers suspended their operations in Syria on Saturday, blaming intensifying violence as troops rained shells down on rebel bastions including Homs where the opposition warned a massacre was looming.
The United States said the decision marked a "critical juncture" for Syria and that it was discussing with its allies the way ahead for a "political transition" as set out in two UN Security Council resolutions.
Syria's neighbour Turkey and Britain also voiced concern with London saying the suspension "calls into serious question" the viability of the entire operation.
On the ground, meanwhile, violence claimed nearly 50 more lives, monitors said, at least 35 of them civilians.
The unarmed observers have been targeted almost daily since deploying in mid-April to monitor a UN-backed but widely flouted ceasefire, and they were likened to "sitting ducks in a shooting gallery" by Susan Rice, the US envoy to the United Nations.
Explaining the suspension, mission head Major General Robert Mood spoke of an escalation in fighting and of the risk to his 300-strong team, as well as the "lack of willingness" for peace by the warring parties.
"There has been an intensification of armed violence across Syria over the past 10 days," Mood said in a statement.
"This escalation is limiting our ability to observe, verify, report as well as assist in local dialogue and stability projects -- basically impeding our ability to carry out our mandate," he said.
"In this high risk situation, UNSMIS (United Nations Supervision Mission in Syria) is suspending its activities," Mood said.
The observers "will not conduct patrols and will stay in their locations until further notice," the general said, adding that "engagement with the parties will be restricted."
Mood said the suspension would be reviewed daily, and that "operations will resume when we see the situation fit for us to carry out our mandated activities."
The UN mission's suspension of operations came two months into its three-month mandate, and after the United Nations accused both sides in the Syria conflict of willingly intensifying the violence.
The United States, Britain and Turkey reacted to the news expressing deep concern.
"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 2042 and 2043," a White House official said.
Resolutions 2042 and 2043 addressed the observers' deployment to Syria under a UN-Arab envoy Kofi Annan's peace plan aimed at "facilitating a Syrian-led political transition" leading to democracy, among other conditions.
In London Foreign Secretary William Hague issued a statement expressing "regret" and said: "This underlines the extent of the deterioration of security and stability in Syria, and calls into serious question the viability of the UN mission."
His Turkish counterpart Ahmed Davutoglu said in Istanbul: "In the event that this observer mission pulls back, there is need for the UN Security Council to immediately do a situation assessment and take a new measure to ensure the humanitarian tragedy does not move onto a next level."
Top opposition Syrian National Council official Burhan Ghalioun said the UN should urgently send peacekeepers. "Today, it is clear that one cannot rely on unarmed observers," he said.
The SNC, meanwhile, warned of a looming massacre in the flashpoint central city of Homs, which it said was besieged by 30,000 troops and pro-regime militiamen.
"Regime forces are escalating their shelling of the city of Homs in an unprecedented way," the SNC said, adding it had contacted members of the "Friends of Syria," asking them to "stop the massacre that is being prepared."
The shelling and shooting targeted several Homs districts and killed at least six people, according to the Syrian Observatory for Human Rights, which said more than 1,000 families were trapped in the city.
The watchdog issued an "urgent call" to UN Secretary General Ban Ki-moon "and all those with a sense of humanity to intervene immediately, in order to put a stop to the continuous shelling."
Observatory chief Rami Abdel Rahman told AFP that "more than 100 people are injured" in Homs and need urgent medical care or "some of them will die."
Violence in Syria has killed more than 14,400 people since an uprising against the regime of President Bashar al-Assad erupted in mid-March 2011, according to the Observatory.
It reported at least another 42 people killed on Saturday, taking the countrywide death toll to over 180 in three days, with at least nine troops killed in clashes with rebels who lost four men.
A 10-year-old Sudanese girl was among those killed in shelling of the Homs province village of Farhaniyeh, and a family of three died when a shell slammed into their home in a district of Damascus province.
Regime troops also fiercely clashed with insurgents in the Homs province rebel bastion of Rastan, where at least five of the troops were killed, the Britain-based Observatory said.
Anti-regime activist Abu Rawan described the shelling of Rastan as "insane" and told AFP cars entering or leaving the town were being shot at with heavy machinegun fire.