Libyan rebel leader Mahmud Jibril headed for the White House Friday as he pressed a diplomatic offensive buoyed by a British invitation to open a first foreign office and the breaking of the loyalist siege of Missrata.

Hours after rebel fighters seized the airport of Libya's third-largest city, sending troops loyal to Muammer Gaddafi fleeing down the coastal road towards the capital Tripoli, Canadian and British warships beat back a naval attack on Missrata's vital seaport, NATO revealed late on Thursday.

Jibril, who serves as the foreign minister of the rebels' National Transitional Council (NTC), was to hold White House talks with US national security adviser Tom Donilon.

US officials would not say whether President Barack Obama would drop by Jibril's meeting with Donilon, a practice sometimes used by the White House for guests for whom protocol does not dictate an official meeting.

Asked by CNN television what he expected from Friday's talks with the US administration, Jibril replied: "We need the recognition."

The Libyan opposition, based in the eastern city of Benghazi, wanted Washington to recognise the body as "the sole legitimate interlocutor of the Libyan people," he said.

Unlike France, Italy, Gambia and Qatar, the United States has not yet recognised the NTC. Jibril told CNN he believed Jordan would recognise the opposition in the coming days.

"All we need is for the world to understand our cause and help us get our legitimate rights realised," he said.

But White House spokesman Jay Carney signalled that Washington, which helped launch the NATO military operation against Gaddafi's forces but has since taken a support role, was not ready to grant full status to the NTC.

"If the question is recognising the (NTC) as the official government of Libya, I think that's premature," he said.

"We think that the council serves and has served as a credible and legitimate interlocutor for the Libyan people, for the opposition."

Jibril warned earlier that the council was facing a "very acute financial problem" and needed help from the US administration.

Last week in Rome, Secretary of State Hillary Clinton said the Gaddafi regime's frozen assets in the United States would be used to help the Libyan people.

Around 30 billion dollars (20 billion euros) in Libyan assets have been frozen in the United States as a result of economic sanctions.

Jibril's visit comes as the Obama administration gradually steps up contacts with Gaddafi's opposition to better understand the movement before deciding on the extent of US assistance.

Defence Secretary Robert Gates said the air war in Libya has cost the United States roughly $750 million (525 million euros) to date, more than the Pentagon's initial estimate of $604 million.

On Thursday, another senior rebel leader, Mustafa Abdul Jalil, met British Prime Minister David Cameron, who invited the NTC to open an office in London, its first foreign mission.

In Libya, Salah Badi, who commanded the rebel assault on Missrata's airport, said rebel positions were now only 10 kilometres (six miles) from Zliten, the next main centre on the 215-kilometre (130-mile) coast road from Misrata to Tripoli.

Canadian and British warships patrolling waters off Libya beat back an attack by an unspecified number of fast-moving small boats on Misrata port hours after the city's airport fell to rebels, NATO said in Brussels.

The Canadian frigate HMCS Charlottetown and British destroyer HSM Liverpool joined a French gunboat not under NATO command to turn back the Gaddafi regime forces early Thursday, the alliance said.

The attack came as rebels celebrated the breaking of the two-month siege of the city.

Separately, a French private security contractor was shot dead and four others arrested in a murky incident at a checkpoint in Benghazi, officials said.

"During a police check in Benghazi last night, five French nationals were detained," French foreign ministry spokesman Bernard Valero said.

"One of them was wounded by a bullet and died overnight in a hospital in Benghazi" while the other four remain in detention, he said, confirming a report by a doctor at the hospital.

The French statement did not say who fired the bullet that killed the contractor in Benghazi, which is far from the front line in the Libyan conflict and which at the moment is a relatively safe city.