A huge blaze raged for a third day at a fuel depot near Tripoli's airport Tuesday, while a paramilitary warplane crashed in Libya's second city Benghazi during fighting with Islamists.

Amid increasing lawlessness and uncertainty, France, Portugal, the Netherlands, Canada and Bulgaria became the latest nations to ship out their citizens or close their embassies in Tripoli.

Italy, meanwhile, denied a report by Libyan authorities that Rome was to send seven fire-fighting planes to combat an inferno that has been blazing since Sunday at an oil depot on the outskirts of the capital.

"Italy continues to evaluate all options for helping Libya, bearing in mind the enormous technical difficulties and the fact that the area in question has been an arena for clashes between militias since July 13," the foreign ministry in Rome said.

The blaze was sparked by rocket fire in clashes between rival militias battling for control of Tripoli international airport.

In Benghazi, General Sagr al-Jerouchi, chief of air operations for dissident ex-general Khalifa Haftar, said it was not immediately clear if the fighter jet had been hit by gunfire or suffered a malfunction before crashing and exploding.

He said the pilot had safely ejected, which was confirmed by a witness who said he saw a parachute open before the plane crashed.

The witness said the warplane had just attacked Islamist positions.

Two weeks of fighting around Tripoli airport and between Islamists and a former general in Benghazi has killed scores of people and prompted several countries to urge their citizens to leave Libya.

The Tripoli fire broke out when a rocket struck a tank containing more than six million litres (1.6 million gallons) of fuel before spreading to a second storage site.

The authorities fear the blaze could spread still further to a natural gas reservoir, where 90 million litres are stored, amid fears a huge fireball could cause carnage over a wide area.

While the oil burns, motorists in Tripoli are suffering severe petrol shortages, as service stations have closed over fears for the safety of staff in light of the fighting.

On Monday, the government appealed to several countries for help, as Italy as well as Greece said their involvement would be contingent on a halt in the fighting.

On Tuesday, the government again called for a ceasefire in the battle for the airport that has left around 100 dead and 400 wounded since July 13.

- Tripoli clashes resume -

Clashes raged on Monday and resumed on Tuesday afternoon after a brief lull.

On Monday, top world leaders urged an immediate ceasefire and called on the UN "to play an essential role in facilitating the political process" to restore stability to Libya.

The clashes, the most violent since the 2011 revolt, started with a July 13 assault on the airport by armed groups, mainly Islamists.

The attackers are battling to flush out fellow former rebels from the hill town of Zintan, southwest of Tripoli, who have controlled the airport for three years.

Weekend fighting in Benghazi, cradle of the 2011 revolution that ousted dictator Moamer Kadhafi, killed dozens of people, mostly soldiers.

Combat erupted Saturday when Islamists attacked the headquarters of a special forces unit near the city centre. One of the few regular army units in Benghazi, it backs an anti-Islamist campaign launched by Haftar in May but has not placed itself under his command.

Since May, clashes have taken place in Benghazi on an almost daily basis.

France was evacuating its nationals by sea while the Netherlands and Portugal announced they were temporarily closing their embassies.

The Portuguese foreign ministry said it had already evacuated its nationals who wanted to leave and those who decided to stay did so at their own risk, the ministry added.

Canada and Bulgaria joined the diplomatic exodus on Tuesday, transporting staff to Tunis.

Several other countries, including Britain, Germany and Egypt, at the weekend advised their nationals to leave immediately.

The United States for its part evacuated its embassy, citing a real risk because of fighting between troops loyal to the Libyan government and Islamists.

The exodus of foreign workers will further hit the strife-torn country, with the health ministry warning of a shortage of medical staff after the Philippines announced it was withdrawing its citizens, including 3,000 doctors and other healthcare workers.