Libyan rebels on Wednesday pinned down Muammar Gaddafi's troops at the airport of the city of Misrata after a night of fierce battles, as the UN chief urged Tripoli to halt attacks on civilians.

The European Union, meanwhile, said it will open an office in the rebel-held city of Benghazi to shore up assistance for the opposition to Gaddafi, even as rebel leader Mahmud Jibril was set to meet with key US lawmakers.

An AFP correspondent reported that the rebels had cornered Gaddafi's troops at the Misrata airport after fighting that raged through the night.

He said the insurgents were on Wednesday morning in control of the northern, eastern and western perimeters of the airport compound, leaving only the south where Gaddafi's troops would face fierce resistance if they wanted to fight their way out.

Insurgent forces captured 40 Grad rockets from the regime troops, whose mortar fire injured 13 rebels, the correspondent said.

It was unclear if any regime fighters were killed or wounded, but rebels said they had captured one Mauritanian mercenary in Gaddafi's employ, although reports could not confirm this.

Rebels had been firing on Gaddafi's forces from a prison in the airport compound, having already gained control of the African market also in the enclosure.

They were reinforced by fighters returning from the front further west, which rebels told AFP was quiet after they had pushed government forces about 15 kilometres (10 miles) from Misrata.

The insurgents were advancing to Dafnia, and were readying to move on Zliten, the next major town on the road to Tripoli.

Misrata had been besieged by Gaddafi forces for close to two months, during which time the only route in or out of the city, Libya's third largest, was by sea via the port.

Should the rebels capture the airport, the humanitarian situation could be drastically eased for the city's 500,000 population, who are facing widespread shortages of food and medical supplies.

Inspired by the uprisings in other Arab nations, rebels have been fighting since mid-February to oust Gaddafi but have met with stiff resistance despite gaining a foothold in the eastern city of Benghazi.

Haj Mohammed, a Misrata rebel commander, said on Tuesday that 'every day we manage to advance along the coastal road toward Zliten. Yesterday 15 kilometres (10 miles), today only two, but the advance is unstoppable'.

Rebels were using shipping containers to shield themselves from loyalist fire, and bulldozers were pushing them forward as the advance continued.

Ahmad Hassan, a rebel spokesman in Misrata, said the insurgents had also 'liberated' areas south and east of the city, killing many Gaddafi troops and seizing a large amount of weapons. Eighteen rebels and civilians were wounded.

The rebel claims could not be immediately verified.

Explosions, meanwhile, were heard from eastern parts of the capital Tripoli for almost an hour on Wednesday morning as jets flew overhead, a witness told AFP.

NATO said that since the alliance took over military operations on March 31 to protect civilians from pro-Gaddafi forces, jets have conducted almost 6,000 sorties, including more than 2,300 strike missions.

Bombs were not dropped during all of those missions, figures showed, as officials insisted again the raids were not aimed at killing Gaddafi, who has ruled Libya for more than four decades.

"All NATO targets are military targets, which means that the targets we've been hitting, and it happened also last night in Tripoli, are command and control bunkers," Brigadier General Claudio Gabellini told reporters on Tuesday.

"NATO is not targeting individuals," he said via videolink from the operation's headquarters in Naples, Italy.

But asked whether Gaddafi was still alive, the Italian NATO general said: "We don't have any evidence. We don't know what Gaddafi is doing right now."

Italian Defence Minister Ignazio La Russa said Gaddafi is a legitimate target if he is inside a military installation.

"If, for example, it's a place from which orders are being issued to strike against civilians then a raid is legitimate," he said in an interview with Il Messaggero on Wednesday.

Gaddafi survived a NATO bombing on May 1 in Tripoli, which killed his second-youngest son, Seif al-Arab, and three of his grandchildren.

UN chief Ban Ki-moon on Wednesday urged Libya's Prime Minister Baghdadi Mahmudi to immediately stop attacks on civilians and called for an 'end of fighting in Misrata and elsewhere'.

"I told him the Libyan authorities must stop attacking civilians, I said there must be an immediate verifiable ceasefire negotiations towards the peaceful resolution of the conflict and unimpeded access to humanitarian workers," Ban told journalists.

The European Union decided to open an office in Benghazi in order to boost its assistance to the anti-Gaddafi movement.

The aim of the new office would be 'to support civil society, to support the interim national council... to support security sector reform, to build on what the people ask us to', EU foreign policy chief Catherine Ashton told the European parliament.

"They want health and education, health-care, security on the borders, the kind of support we are able to give them and want to give them."

Her statement came the same day rebel leader Jibril was to meet in Washington with key US lawmakers, including senator John Kerry, a Democratic ally of the White House who chairs the Senate Foreign Relations Committee.

"The Foreign Relations Committee and the American people are eager to learn more about the opposition movement in Libya and Mahmud Jibril is well positioned to answer our questions," Kerry said in a statement on Tuesday.