The US has said it is consulting international partners over steps to take over Syria, after UN observers suspended their activities.
Norwegian Gen Robert Mood, the head of the UN Stabilisation Mission (UNSMIS), said observers would cease operations because of the escalating violence.
But he said the mission remained committed to ending the violence.
The UK foreign secretary said the decision called into question the mission’s viability.
William Hague blamed the government of President Bashar al-Assad for the worsening situation.
In a statement, he said the suspension of patrols and restriction of staff movements “underlines the extent of the deterioration of security and stability in Syria, and calls into serious question the viability of the UN Mission”.
Activists reported on Saturday that at least 60 people had been killed around the country, with the worst violence in areas around Damascus, where they said 10 people were murdered in the town of Saqba.
At least seven people were killed in Douma, an eastern suburb of the capital Damascus, while at least 18 others died in violence elsewhere, the UK-based Syrian Observatory for Human Rights reported.
The opposition Syrian National Council also warned of a looming massacre in Homs which it says is besieged by 30,000 troops and pro-regime militiamen.
Syria restricts access to foreign media, and reports of killings are difficult to verify.
US White House spokesman Tommy Vietor urged Syria to abide by international mediator Kofi Annan’s six-point peace plan.
“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.
Arab League spokesman Ahmad Bin Hilli told the BBC the decision was a temporary one, and the operation could be cancelled only by the UN in consultation with the Arab League.
Announcing the suspension of UNSMIS operations, Gen Mood said the suspension would be “reviewed on a daily basis” and that a “return to normal operations remains our objective”.
The UN Security Council’s five permanent members will consider the next steps for the observer mission when Gen Mood briefs them on the situation in Syria on Tuesday.
The mission’s 298 military observers and 112 civilian staff were sent to Syria to verify the implementation of the Annan plan, which included a ceasefire.