The White House said on Tuesday there was no evidence so far of a promised Syrian army pullback and the United States would work with international partners on "next steps" if Damascus failed to meet its commitments under a UN-sponsored ceasefire deal.
"Leaders of the Assad regime ... make a lot of promises," White House spokesman Jay Carney told reporters aboard Air Force One as President Barack Obama headed for Florida.
"Those promises overwhelmingly turn out to be empty."
Carney spoke as international envoy Kofi Annan said a plan designed to stem 13 months of conflict in Syria was still on the table, despite signs the deal was in peril.
Annan said he had information the Syrian military was withdrawing from some areas but moving into others.
"We have seen no evidence thus far of any pullback," Carney said when asked about the Syrian foreign minister's assertion that some troops had been removed from cities.
"We have seen much evidence of further brutality and oppression against innocent civilians.
"We are waiting for the assessment that special envoy Kofi Annan put forward today at the United Nations and we will certainly work with, in the aftermath of that, with our partners and others on next steps with regards to Syria," Carney said.
Carney gave no indication of what further moves might be under consideration against Syria over its crackdown on the opposition.
But he said, "We would certainly hope that the United Nations Security Council would evaluate the situation in Syria if in fact Mr Annan finds the Assad regime has not abided by its own commitment to begin withdrawal by today."