Leaders from Britain, the US, France, and Arab countries are due to meet in Paris to discuss military action in Libya under a new UN resolution.
The UN Secretary General Ban Ki-moon, who is also attending, said the world must “speak with one voice” on Libya.
Thursday’s Security Council resolution authorised “all necessary measures” to protect civilians.
US President Barack Obama said pro-Gaddafi forces must stop attacking rebel areas or face military action.
“Gaddafi must stop his troops from advancing on Benghazi, pull them back from Ajdabiya, Misrata and Zawiya and establish water, electricity and gas supplies to all areas,” he said on Friday.
But in line with the resolution, Mr Obama ruled out using US ground forces.
Early on Saturday, explosions and the sound of jets were reportedly heard close to Benghazi, but there were few details and the reports could not be confirmed.
On Friday Col Muammar Gaddafi’s government declared a unilateral truce but there were reports that government offensives in rebel-held towns were continuing.
The BBC’s Kevin Connolly in Tobruk says the rebels have repeated their call for immediate air strikes against the Gaddafi forces.
The Security Council vote called for an immediate ceasefire and endorsed the use of force – including a no-fly zone aimed at preventing pro-Gaddafi forces from bombing rebel-held towns.
The British and French, along with some Arab allies, are expected to play a leading role in any initial air strikes.
The French ambassador to the UN, Gerard Araud, told the BBC that he expected military intervention within hours of Saturday’s summit.
UK Prime Minister David Cameron – who is attending the meeting – has said British war planes are being moved to bases in the region.
“The clock is ticking and we must be ready to act quickly,” Mr Cameron said.
The summit will be hosted by French President Nicolas Sarkozy. US Secretary of State Hillary Clinton will represent the Obama administration.
BBC diplomatic correspondent Bridget Kendall says it is a chance to demonstrate to the Gaddafi government, and to the rest of the world, the strength of outrage at his actions and the determination to stop him.
The US Navy is deploying additional warships to the Mediterranean to support possible military action.
The commander in charge of planning for the no-fly zone will be US Adm Samuel Locklear, the BBC’s Mark Urban has learned.
Denmark and Canada have said they will supply fighter jets, with Italy, Spain and France making air bases available.
Saudi Arabia, Qatar, the UAE and Jordan could be among Arab states taking part.
Nato is already providing 24-hour surveillance over Libya with its Awacs planes.
Secretary-General Anders Fogh Rasmussen said Nato was “completing its planning to be ready to take appropriate action in support of the UN resolution as part of the broad international effort.”
The US ambassador to the UN, Susan Rice, told CNN that Col Gaddafi was already in violation of the UN Security Council resolution.
But Libyan Deputy Foreign Minister Khaled Kaim has insisted that his government is observing the ceasefire, and invited foreign observers to monitor it.
Early on Saturday he accused the rebels of breaching the truce by attacking government forces south of the rebel stronghold of Benghazi, in the east of the country.
Col Gaddafi has ruled Libya for more than 40 years.
An uprising against him began last month after long-time leaders of neighbouring Tunisia and Egypt were toppled.
Al-Jazeera reported him on Friday as saying that the UN resolution was “blatant colonialism” that “does not have any justification”.
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