Hillary Clinton’s comments came after Russia and China renewed opposition to tougher UN Security Council action, reports the BBC.
UN Secretary General Ban Ki-moon has repeated a warning that Syria could be moving towards “catastrophic” civil war, in the wake of the Houla massacre.
Rebel commanders are split on whether to abandon a ceasefire if Syrian forces do not withdraw to barracks.
Colonel Qassim Saadeddine of the Free Syrian Army (FSA) in Homs said that if there was no government response by Friday lunchtime, the FSA would consider itself “no longer bound” by the plan.
But the FSA head, General Riyad Asaad, later denied the deadline existed.
Instead, he urged peace envoy Kofi Annan to issue a statement declaring his peace plan to have failed.
The BBC’s Paul Wood, who has just returned from three weeks inside Syria, says there is no ceasefire holding on the ground.
Syrian President Bashar al-Assad has come under intensified pressure to adhere to the ceasefire plan since the Houla massacre, in which more than 100 people – many of them children – died.
On Thursday, a Syrian general gave what he said were the preliminary results of an ongoing investigation.
General Qasem Jamal Suleiman said “terrorist armed groups” carried out the killing to persuade the outside world that Syria was sliding into civil war and to trigger external intervention.
Mrs Clinton, speaking on a visit to Denmark, said the case for military intervention was growing stronger every day.
“[The Russians] are telling me they don’t want to see a civil war. I have been telling them their policy is going to help to contribute to a civil war,” she told an audience in Copenhagen.
The BBC’s State Department correspondent, Kim Ghattas, says Mrs Clinton’s words will be comforting to Assad but will dismay the rebels.
However, our correspondent says both sides will need to keep in mind that conditions for international action can change very quickly.
The office of UK Prime Minister David Cameron has said Britain “will not abandon” the people of Syria following the Houla massacre, and “all the options on the table” were being considered.
Syria’s foreign ministry said the Houla massacre was carried out by armed gangs
Ban, speaking at a conference in Turkey, said UN monitors had not been sent to Syria “just to bear witness to the slaughter of innocents”.
“The massacre of civilians of the sort seen last weekend could plunge Syria into catastrophic civil war – a civil war from which the country would never recover,” he said.
In a video released online, Col Saadeddine said the government had to “implement an immediate ceasefire, withdraw its troops, tanks and artillery from Syrian cities and villages”.
“It should also allow immediate humanitarian aid to all affected areas and free all detainees… The regime should also enter into a real and serious negotiation through the United Nations to hand over power to the Syrian people,” he went on.
But Colonel Asaad, speaking to al-Jazeera by phone from Turkey, insisted the FSA was “committed to the Kofi Annan plan and committed to international resolutions and implementing this plan”.
“There is no deadline; however, we hope that Kofi Annan will issue a statement to announce the failure of this plan,” he said.
Source: BBC News