Libyan leader Muammar Gaddafi''s forces pushed into the rebel-held city of Benghazi on Saturday, defying world demands for an immediate ceasefire and forcing rebels to retreat.

A Libyan rebel spokesman said Gaddafi''s forces had entered the city while a Reuters witness saw at least one explosion near the rebel movement''s headquarters in the city.

"They have entered Benghazi from the West. Where are the Western powers? They said they could strike within hours," rebel military spokesman Khalid al-Sayeh told Reuters.

The fighting came despite a pledge by Libya''s government that it was observing a ceasefire and as France predicted imminent military action by the West.

"Everything is ready (to act) but the decision is now a political one. It''s clear we have to move quickly," a French government source said, hours before France was due to host an international meeting to discuss military intervention.

The Libyan government denied its forces were in action in or around Benghazi. A government spokesman said they were observing the ceasefire, blaming rebels for attacks.

As explosions shook Benghazi, rebel fighters said they were being forced to retreat from the outskirts of the city where the revolt against Gaddafi began a month ago.

A fighter jet was shot down over Benghazi on Saturday.

"I saw the plane circle around, come out of the clouds, head towards an apparent target, and then it was hit and went straight down in flames and a huge billow of black smoke went up," Reuters correspondent Angus MacSwan said.

"It seems it was attacking the Benghazi military barracks."

Troops bomb road to Benghazi

Rebels said Libyan jets had bombed the road to Benghazi airport and elsewhere on the outskirts.

The Libyan advance into Benghazi pre-empted an international meeting hosted by France on Saturday to discuss military intervention in Libya. The meeting will be attended by US Secretary of State Hillary Clinton and Arab leaders.

Libya had declared a unilateral ceasefire on Friday after the UN Security Council authorised a no-fly zone over Libya.

But the United States accused Gaddafi of defying international demands for an immediate ceasefire, and France''s UN envoy predicted military action within hours of the Paris meeting on Libya on Saturday.

"No justification"

Within hours of President Barack Obama saying the terms of a UN resolution meant to end fighting in Libya were non-negotiable, his UN envoy Susan Rice, asked by CNN whether Gaddafi was in violation of these terms, said: "Yes, he is."

Gaddafi said there was no justification for the UN resolution.

"This is blatant colonialism. It does not have any justification. This will have serious consequences on the Mediterranean and on Europe," he said in comments reported by Al Jazeera television.

France, which along with Britain has been leading a drive for military intervention, will host a meeting on Saturday on Libya which will be attended by US Secretary of State Hillary Clinton and Arab leaders.

Obama made clear any military action would aim to change conditions across Libya - rather than just in the rebel-held east - by calling on Gaddafi''s forces to pull back from the western cities of Zawiyah and Misrata as well as from the east.

"All attacks against civilians must stop," Obama said, a day after the UN Security Council passed a resolution authorising international military intervention.

"Gaddafi must stop his troops from advancing on Benghazi, pull them back from Ajdabiya, Misrata and Zawiyah, and establish water, electricity and gas supplies to all areas. Humanitarian assistance must be allowed to reach the people of Libya...

"Let me be clear, these terms are not negotiable... If Gaddafi does not comply... the resolution will be enforced through military action."