The White House is consulting with international partners on "next steps" in the Syrian crisis after UN observers suspended operations there in response to violence that has escalated despite a ceasefire deal.
"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.
US officials have voiced growing skepticism about Syrian president Bashar al-Assad's willingness to comply with the tattered plan brokered by international mediator Kofi Annan
Washington and its Western allies have shown no appetite for a Libya-style military intervention even as Moscow has helped shield Dr Assad from tougher UN measures.
"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," Mr Vietor said.
But he provided no specifics. "The sooner this transition takes place, the greater the chance of averting a lengthy and bloody sectarian civil war.”
The head of the observer mission, Gen Robert Mood, said today: "The observers will not be conducting patrols and will stay in their locations until further notice".
He said the intensification in violence was impeding the 300 unarmed UN monitors from carrying out their mandate to observe an April 12th ceasefire deal.
"This suspension will be reviewed on a daily basis. Operations will resume when we see the situation fit for us to carry out our mandated activities," Gen Mood said.
Many hundreds of people, including civilians, rebels and government forces, have been killed in the two months since Mr Annan's ceasefire deal was supposed to come into effect.
Last week shots were fired at a car carrying UN observers after they were turned away from the town of Haffeh by angry Assad supporters who threw stones and metal rods at their convoy, a spokeswoman for the monitors said.
Diplomats say Gen Mood is expected to brief the United Nations Security Council on Monday or Tuesday about the unrest in Syria, which head of UN peacekeeping said this week was now in the throes of full-scale civil war.
The five veto-wielding Security Council members are expected to have discussions on the sidelines of a G20 summit in Mexico, which starts on Monday, aimed at breaking deadlock on the issue of international sanctions against Syria.
Syria's foreign ministry said it had been informed of Gen Mood's decision yesterday evening and told him it understood his concern for the safety of the monitors, blaming the attacks on rebels fighting government forces.
Since the start of the ceasefire deal the "armed terrorist groups" - the label Damascus gives to anti-Assad fighters - had escalated their "criminal activities, which have often targeted the UN observers", a foreign ministry statement cited by state news agency SANA said.
Reuters