U.S. President Barack Obama and British Prime Minister David Cameron have held talks on the ongoing crisis in Libya.
Obama and Cameron agreed Muammar Qaddafi must leave Libya as soon as possible.
The White House said the two also discussed a potential no-fly zone over Libya.
Earlier, U.S. Secretary of State Hillary Clinton said it was up to the United Nations to decide whether to take such action.
British Foreign Secretary William Hague said Britain and France were working on a Security Council resolution on a no-fly zone.
"What we are working on is elements of a resolution on a no-fly zone. We are working on that with France at the United Nations Security Council," Hague said. "That is still contingent to planning, of course. As I explained yesterday, there must be a demonstrable need that is accepted broadly by the international community."
Meanwhile, forces loyal to Qaddafi used rockets, tanks and warplanes in a bid to retake territory under control of rebels hoping to end the Libyan leader's 41-year rule.
Intense fighting was reported in Zawiya, the closest rebel-held city to Tripoli.
Residents quoted by Reuters said up to 50 tanks were taking part in the siege there.
The government said its troops were mostly in control of the city.
Elsewhere, government warplanes targeted rebel positions around Ras Lanuf and Bin Jawad.
The two strategic cities are about 60 kilometers apart on the Mediterranean coast in the east of the country, where rebel control is strongest.
In Strasbourg, France, members of the Libyan opposition met the EU foreign policy chief Catherine Ashton and later today are due to speak at the European Parliament.
Mahmoud Jebril, head of the crisis committee of the National Libyan Council, said the EU should recognize the council as the legitimate representative of the Libyan people.
compiled from agency reports