Susan Rice, the US ambassador to the United Nations spoke ahead of a Security Council vote due today on the no fly zone being implemented to halt Col Gaddafi's attacks on rebels.
While China, Russia, Germany and other members of the 15-member council have expressed opposition or doubts about military action in Libya, the United States said even stronger measures than a no-fly zone may be needed.
A draft resolution which includes a no-fly zone has been put forward for a vote on Thursday. Britain, France and the United States had acted "to put pressure on the council to act quickly and decisively," said one diplomat, announcing the move.
Final talks will be held before a vote and the draft could still be changed, diplomats from the three countries acknowledged.
Russia made a counter proposition for a ceasefire resolution as a stop-gap before a full sanctions measure, the country's envoy Vitaly Churkin told reporters. But this did not get enough support for a vote.
Miss Rice however said the council was discussing "a range of actions".
"We need to be prepared to contemplate steps that include, but perhaps go beyond, a no-fly zone at this point, as the situation on the ground has evolved, and as a no-fly zone has inherent limitations in terms of protection of civilians at immediate risk," she said.
Gaddafi's forces have inflicted several defeats on rebels in recent days and US Secretary of State Hillary Clinton also hinted at a hardening Washington stance earlier when she said the United States wanted a vote by the end of Thursday.
France and Britain had led the demands for a no-fly zone and French President Nicolas Sarkozy wrote to the heads of state or government of all the other council members seeking urgent backing for the measure.