Pro-Gadhafi forces were reported, meanwhile, to be holding out against the allied campaign to break their hold on the ground while enforcing a no-fly zone.

As the assault unfolded late Sunday, an explosion thundered from Gadhafi’s personal compound in Tripoli, and a column of smoke rose above it, suggesting that the allied forces may have struck either his residence there or the nearby barracks of his personal guards.

Unnamed Western officials were quoted in various news reports as saying the building was a military command and control centre.

Rebel fighters trying to retake the eastern town of Ajdabiya said they were driven back on Monday by rocket and tank fire from government loyalists still controlling entrances to the city. Dozens of fighters retreated to a checkpoint around 12 miles north of the Ajdabiya, and said at least eight others had been killed during the day’s fighting, including four who had been standing in a bloodied pickup truck that the fighters showed to reporters.

There were conflicting reports about whether the allies had attacked loyalist forces in the town. While planes had been heard overhead, the rebel fighters said there appeared to have been no attack on the pro-Gadhafi forces holding the entrance to Ajdabiya on the coastal highway leading north to Benghazi.

Ajdabiya is a strategically important town that has been much fought over, straddling an important highway junction and acting as a chokepoint for forces trying to advance in either direction. The Ajdabiya retreat appeared to have thrown the rebels into disarray, with a  rebel commander at the checkpoint trying to marshal his forces, using a barely functioning megaphone, but few of the fighters heeding his exhortations.

In the western city of Misurata, forces loyal to Gadhafi were still at large and were using civilians from nearby towns as human shields, Reuters reported. But there was no immediate confirmation of that report.

Journalists taken by the Gadhafi government to visit the site shortly after the blast said they saw a bomb-damaged building that appeared to be an administrative centre rather than a military barracks or a Gadhafi residence, although its exact nature could not be definitively confirmed. No casualties were reported, though government spokesman Moussa Ibrahim called it “a barbaric bombing.”

Asked about the explosion, Vice-Adm William E Gortney said in a Washington news conference that the United States was not trying to kill the Libyan leader. “At this particular point I can guarantee that he’s not on a targeting list,”he said, saying that the US military was working to weaken his forces’ capacity rather than remove him