Pro-Gaddafi forces have been gaining ground eastwards along the coast Western powers are discussing how to enforce a no-fly zone over Libya after the passage of a UN resolution backing "all necessary measures" to protect civilians, short of an occupation.
France said there could be air strikes "within hours", though the details and timing of any action remain unclear.
Forces loyal to Col Muammar Gaddafi have been advancing eastwards towards the rebel stronghold of Benghazi.
They were also said to be bombarding the city of Misrata on Friday.
Col Gaddafi has promised to retake Benghazi, saying his forces would show "no mercy".
Meanwhile, Libya has closed its airspace to all traffic, Europe's air traffic control agency says.
Nato is due to discuss on Friday what role the alliance might play.
French government spokesman Francois Baroin said on Friday morning that strikes could take place "rapidly" and "within a few hours". But he added: "You will understand that there's no question of talking as early as this morning about when, how, which targets or in which form."
'Unfair'
It is not thought that the US would be involved in the first strikes. The British and French, along with some Arab allies, are expected to play a leading role. Norway has said it will also participate.
Qatar will take part in international efforts to protect civilians, Qatar's official news agency reported, though it was not clear if this included military operations.
Continue reading the main story Libya action contributors
- France
- UK
- US
- Qatar
- Norway
- Denmark (subject to parliamentary vote)
- Poland (logistical support)
The UN resolution is so broad that it allows military action against all threats to civilians, and could involve bombing Col Gaddafi's forces on the ground if deemed necessary.
Rebel forces in Benghazi reacted with joy to the UN resolution, but a government spokesman condemned UN "aggression". One of Col Gaddafi's sons, Saif al-Islam, said the resolution was "unfair" as Libya had not been bombing civilians.
Following the UN vote, US President Barack Obama called the French and British leaders to discuss the next move. They said Libya had to comply immediately with the resolution.
"Given the critical situation on the ground, I expect immediate action on the resolution's provisions," said UN Secretary-General Ban Ki-moon.
BBC diplomatic correspondent Jonathan Marcus says signals from Paris that air operations could be imminent may be an attempt to keep Col Gaddafi guessing.
US officials said an attempt to ground Col Gaddafi's air force could begin on Sunday or Monday.
'Strong message'
The UK, France and Lebanon proposed
Security Council Resolution 1973, with US support.
In New York, the 15-member Security Council voted 10-0 in favour, with five abstentions.
Russia and China - which often oppose the use of force against a sovereign country as they believe it sets a dangerous precedent - abstained rather than using their power of veto as permanent members.
The US ambassador to the UN, Susan Rice, said: "This resolution should send a strong message to Colonel Gaddafi and his regime that the violence must stop, the killing must stop and the people of Libya must be protected and have the opportunity to express themselves freely."
But Germany, which abstained, will not be contributing to the military effort. Foreign Minister Guido Westerwelle said his government sees "considerable dangers and risks" in military action against Col Gaddafi.
The Russian military also said it would not participate in any military action, Russian news agency Interfax reported.
Chinese foreign ministry spokeswoman Jiang Yu said Beijing had "serious reservations" about the resolution but did not veto it "in view of the concerns and stance of the Arab countries and African Union and the special circumstances that currently apply in Libya".
The Libyan military has warned that civilian and military activities in the Mediterranean would become "the target of a Libyan counter-attack" following any foreign operation.
Following the toppling of the long-time leaders of neighbouring Tunisia and Egypt earlier this year, Libyan protesters started to demand that Col Gaddafi step down after more than 40 years of autocratic rule.
