Britain on Thursday hailed a "change in the position" of the United States on Libya and urged immediate action to stop Moummar Gaddafi's forces, piling on pressure for a UN vote on a no-fly zone.
"There has been a significant change in the position of the White House," Alistair Burt, a British Foreign Office minister with special responsibility for the Middle East, told BBC radio. "We obviously appreciate that the United States does realise the urgency of the situation."
The situation in Libya "makes it imperative that something is done and something is done today," he said, adding he hoped there would be a vote on a resolution at the UN Security Council in New York later on Thursday.
France, Britain and the United States were pressing for the Security Council vote on a no-fly zone to halt Libyan leader Gaddafi's advance on rebel strongholds on the east.
The French and British have led calls for a no-fly zone, and on Wednesday the United States joined the European powers in putting forward a draft resolution which included the measure.
The US envoy to the United Nations, Susan Rice, told reporters at the UN that a no-fly zone was one of a "range of actions" that should be contemplated.
Rice warned that a no-fly zone "has inherent limitations in terms of protection of civilians at immediate risk".
Russia, Germany and other members of the 15-member Security Council have expressed opposition or doubts about military action in Libya.