Susan Rice said Annan, the Arab League ambassador, had received a letter from Syria's foreign minister on Sunday with the 10 April pledge.
Annan had urged the Syrian government to start the withdrawal immediately and to move no further into populated areas, she said, and "that commitment was provided".
However, opposition groups said the Syrian government had sent troops, backed by tanks, into rebellious areas, hunting down activists and torching and bulldozing houses. Fierce clashes were said to be going on in the centre of Homs, and troops reportedly also moved into the southern town of Dael in search of activists.
Syria's UN ambassador, Bashar Ja'fari, said earlier the 10 April deadline was set "by common accord" between Annan and the Syrian government. He reiterated his government's complete support for Annan's six-point plan to end the year-long Syrian crisis.
But Rice, the current security council president, expressed scepticism about Syria's commitment, saying Damascus had made and broken promises over many months.
"We have seen commitments to end the violence followed by massive intensifications of violence," Rice said. "So the United States, for one, would look at these commitments and say, yet again, the proof is the actions, not in the words."
She added: "Past experience would lead us to be sceptical and to worry that over the next several days, rather than a diminution of the violence, we might yet again see an escalation of the violence. We certainly hope that is not so. We hope the Syrian authorities will implement the commitments they made without condition or codicils."
Annan's plan to end Syria's crisis calls for an immediate withdrawal of troops and heavy military equipment from populated areas, followed by an overall ceasefire, first by government forces and then by opposition fighters to pave the way for talks by all Syrian parties on a political solution.
It includes an immediate, daily two-hour halt to fighting so humanitarian aid can reach suffering civilians, and unhindered access for humanitarian groups and the media.
Rice stressed that the Syrian agreement was limited to the pullout of troops and equipment from cities and towns. She said Annan was expecting details from the Syrian government "very shortly" on the other aspects of the plan.
One of the key issues is trying to unite the many different opposition factions under a single umbrella. Rice said Annan's deputy, Nasser al-Kidwa, had had "constructive exchanges with the opposition to urge them to cease their operations within 48 hours of a complete cessation of government hostilities".