Western powers have destroyed nearly a third of Libyan leader Muammar Gaddafi's military power since launching a military campaign against him last month, NATO officials said on Tuesday.

The area around the Libyan city of Misrata - the only major town in western Libya where a revolt against Gaddafi that began seven weeks ago has not been crushed - was the number one priority of NATO air strikes for now, they said.

NATO took command of operations in Libya from a coalition led by the United States, Britain and France on March 31 and is enforcing a no-fly zone ordered by the United Nations and launching air strikes on Gaddafi's forces to shield civilians.

"The assessment is that we have taken out 30 percent of the military capacity of Gaddafi," Brigadier General Mark van Uhm, a senior NATO staff officer, told a news briefing in Brussels.

Over the last day, air strikes around Misrata hit Gaddafi's tanks, air defence systems and other armoured vehicles, he said.

Near Brega in the east, where intense fighting continued for a sixth day on Tuesday, NATO aircraft struck a rocket launcher, as well as ammunition stores in other areas, he said.

NATO-led air power is holding a balance in Libya, preventing Gaddafi's forces overrunning the seven-week old revolt, but is unable for now to hand the rebels outright victory.

NATO countered criticism by the insurgents that Western air power has become less effective since the alliance took control, saying the military presence in Libyan skies had been maintained.

However, Van Uhm said, Gaddafi's use of civilians as human shields and hiding his armour in populated areas was curbing NATO's ability to hit targets.

"The operational tempo remains, but we have seen a change of tactics (from Gaddafi)," he said. "When human beings are used as shields we don't engage."

He confirmed that NATO air strikes had killed several civilians in the town of Brega in recent days, but gave no estimate for the number of victims.

Rebel leaders said on Saturday that a NATO-led strike had killed 13 fighters as they tried to take control of Brega, but called the incident an unfortunate mistake and urged Western powers to continue their campaign.

Van Uhm said NATO forces had acted in self-defence after rebels fired in the air in celebration. "They have moved those enthusiastic people away from the frontline, so celebratory firing is not used anymore," he said.

Addressing NATO efforts to maintain an arms embargo against Gaddafi, Van Uhm said there had been no violations so far, but commercial shipping continued to function in Libyan ports.

He said he had no information about a Libyan ship that docked at a government-controlled port with a cargo of imported gasoline on Tuesday apparently after crossing a cordon of NATO vessels enforcing international sanctions.

Since the beginning of the military campaign against Libya, 30 percent of Libyan leader Muammar Gaddafi's forces' power has been eliminated, a senior NATO military officer said on Tuesday.

"The assessment is that we have taken out 30 percent of military capacity of pro-Gaddafi forces," Brigadier General Mark van Uhm told a press briefing at NATO headquarters.

The officer also said that NATO aircraft flew 150 sorties and performed 58 "strike sorties" over Libya on Monday.

During "strike sorties," NATO planes carry bombs or missiles, but do not necessarily drop their payload because of bad weather or human shields near the targets.

NATO aircraft fired ammunition to hit targets on the ground in 14 "strike sorties" on Monday, mostly in Misrata and Brega, where rebels are fighting fiercely with pro-Gaddafi forces, van Uhm said.

"I think we have maintained very high operational tempo," he stressed.

Van Uhm said that all sorties over Libya were now commanded by NATO, which took over full command of military operation on March 31 from the United States, countries can still use their national military assets in support of humanitarian aids to Libya, but need coordinate their operations with NATO.

"Nothing is happening without NATO knowing, being coordinated," he said.