UNITED NATIONS — The United States said Tuesday the U.N. Security Council must respond urgently and seriously if Syria fails to keep its pledge to halt offensive military operations by April 10.

U.S. Ambassador Susan Rice told reporters that Syrian forces have been continuing offensive operations and the United States “is concerned and quite skeptical that the government of Syria will suddenly adhere to its commitments.”

President Bashar Assad’s government has agreed to international envoy Kofi Annan’s peace plan and the April 10 deadline to halt the violence and pull troops and heavy weapons out of towns and cities.

Bashar’s regime has been using force for the past year to try to put down what began as a peaceful civilian uprising.

A Syrian government official on Tuesday said troops began pulling out from some calm cities and heading back to their bases. But the claim could not immediately be verified, and activists near the capital Damascus denied troops were leaving their area.

Western leaders have been skeptical of Assad’s promises, because he hasn’t followed through in the past.

“What we have seen since April 1 is not encouraging,” Rice said.

If the Syrian government uses the window before April 10 to intensify the violence rather than de-escalate attacks, “it would be most unfortunate, and it would be certainly our view that the Security Council will need to respond to that failure in a very urgent and serious way.”

Under Annan’s plan, the Syrian government’s withdrawal of troops and military equipment would be followed by an overall cease-fire — first by government forces and then by opposition fighters — to pave the way for talks. The plan also calls for an immediate daily two-hour halt to fighting so humanitarian aid can reach suffering civilians.

Rice said “it’s the hope — although I wouldn’t say the expectation — of members of the council that indeed when we review the situation after April 10, the violence will have ceased on the part of the government and we will be in a realm of considering how the council can reinforce that halt to the violence.”

Annan told the council that if Syria meets the deadline, and this can be verified, then the opposition would have 48 hours to wind down its military activities so there would be a complete cessation of hostilities, his spokesman Ahmad Fawzi said Tuesday.

The United States and France circulated a draft presidential statement to the Security Council on Tuesday supporting Annan, the joint U.N.-Arab League envoy. France’s U.N. Ambassador Gerard Araud said it could be approved Wednesday or Thursday.

Rice said the draft would “underscore the central importance of the Syrian government adhering to its commitment to halt all offensive actions by April 10.”

If Syria doesn’t halt the violence, she said the United States will consult with other Security Council members on “appropriate next steps.”

Russia and China have vetoed two Security Council resolutions condemning Assad’s crackdown on protesters and have ruled out even mentioning the possibility of future council action against Syria.

But the two countries have strongly supported Annan’s mission and the deadline.

If the Syrian regime fails to halt the violence by the target date, Rice expressed hope that the climate in the council would improve and all 15 member would “see the wisdom of delivering not just a strong message but strong actions that might change the calculus of the government in Damascus.”

Such actions could include an arms embargo and economic and political sanctions.

Russia has a keen interest in seeing Annan’s plan succeed, given Moscow’s role as Assad’s key ally, and urged Damascus to quickly comply with his proposal.

Russia’s Foreign Ministry said in a statement Tuesday that it had been informed that Damascus had begun fulfilling its obligations, but it gave no details.