TRIPOLI/MISRATA, Libya (Reuters) - NATO launched a fourth night of airstrikes on Tripoli Thursday, leaving smoke rising from Libyan leader Muammar Gaddafi's compound, after the United States said a cease-fire offer from Libya was not credible.
Several large explosions rocked Tripoli late Thursday night and a column of smoke was seen rising from Gaddafi's Bab al-Aziziyah base, a Reuters correspondent said.
Forces loyal to Gaddafi launched the heaviest bombardment on the rebel-held city of Misrata for days earlier Thursday as Western leaders gathered for a Group of Eight summit in the French seaside resort of Deauville.
Rebel spokesmen in Misrata, scene of some of the fiercest fighting in Libya's three month old conflict, said a mortar attack there killed three rebels.
Suleim Al-Faqih said the Misrata clashes started when rebels attacked Gaddafi forces who were using an excavator to dig a trench to block a road.
"We fired on them and advanced. They fell back and started firing mortars," he said.
At a news conference in Misrata late Thursday, Fathi al Bashaagha, a member of the town's military council, said rebel forces advanced four km (two miles) west Thursday and destroyed weapons depots belonging to Gaddafi loyalists before falling back to their front line on Misrata's outskirts.
The military council said it had no immediate plans to advance on Zlitan, the next major town west on the road to Tripoli, which is currently held by Gaddafi loyalists.
"We're waiting for Zlitan to start the battle, then we will be at their call," Bashaagha said. "We think that will be in the next few days."
Spain said it was one of several foreign states contacted by Libyan Prime Minister Al-Baghdadi Ali Al-Mahmoudi with an offer of an immediate cease-fire.
But White House deputy national security adviser Ben Rhodes, speaking in Deauville, said the United States did not see the new Libyan cease-fire offer as credible.
Libya was not complying with U.N. demands and its forces were still attacking population centres, so the United States would continue with the military campaign, he told reporters.
GADDAFI DEPARTURE ESSENTIAL
At a Tripoli news conference, Al-Mahmoudi said the offer was based on an existing African Union roadmap to resolve the conflict, which, crucially, does not mention Gaddafi's future.
"Libya is serious about a cease-fire," he said.
But he added: "The leader Muammar Gaddafi is the leader of the Libyan people; he decides what the Libyan people think. He is in the hearts of the Libyan people."
The rebels said they wanted any government initiative to include the Libyan leader's departure as a first step.
"We welcome any initiative which starts with the departure of Gaddafi, his sons and his regime from Libya," Mustafa Abdel Jalil, head of the rebel Transitional National Council, said on Al Jazeera television.
Gaddafi's security forces cracked down ferociously when thousands of Libyans rebelled against his rule. NATO missiles and warplanes have been bombing targets in Libya for two months under a U.N. mandate to protect civilians from attack.
Rebels now control the east of the country, around their main stronghold of Benghazi, and pockets of land in the West.
But the conflict has reached stalemate on the ground, with the rebels unable to advance towards Tripoli and NATO powers -- wary of getting sucked into new conflicts after their experience in Iraq and Afghanistan -- refusing to put troops on the ground.
Nevertheless, Western officials say they are confident that they are gradually loosening Gaddafi's grip on power through a combination of sanctions and military and diplomatic pressure.
British diplomatic sources said many of Gaddafi's senior commanders had stopped using their telephones for fear that their calls were being listened to, and there was a sense that the "regime was feeling the pressure and beginning to fracture."
Libya's ambassador to the European Union and Benelux countries, Al Hadi Hadeiba, earlier defected after lengthy talks with EU officials, the Union said.
Britain's defence ministry said its Typhoon and Tornado aircraft had used Paveway guided bombs to attack a military vehicle depot at Tiji, in western Libya, which was being used to support attacks on the rebel-held Western mountains region.
British officials said Britain's helicopters were now ready to fly sorties over Libya. British ministers gave clearance in principle for the use of Apache helicopters Thursday.
France has already said it will deploy attack helicopters.
French President Nicolas Sarkozy, the most hawkish Western leader on Libya, is hosting the G8 summit and is expected to use it to press other powers to ramp up pressure on Gaddafi.
Attempts to build a consensus at the summit on Libya may be prevented by Russia, which opposes the NATO bombing.
Gaddafi denies that his troops target civilians and say his security forces were forced to act to put down a rebellion by criminals and members of al Qaeda.
(Reporting by Joseph Logan in Tripoli, Mohammed Abbas in Misrata, Hamid Ould Ahmed in Algiers, Tracy Rucinski in Madrid, Sami Aboudi in Cairo, Missy Ryan in Washington, Tim Castle in London, Keith Weir in Deauville, Joseph Nasr in Berlin and Steve Gutterman in Moscow; Writing by Christian Lowe and Jan Harvey; Editing by Angus MacSwan)