Fighting at the Libyan capital's only working international airport killed nine people Monday, officials said, after militiamen attacked it in an attempt to free colleagues held at a jail there.

Although Mitiga airport was evacuated when the clashes erupted, it was not yet known whether the casualties were all fighters or included civilians.

"Initial toll following the fighting on the perimeter of Mitiga airport: 5 dead at Mitiga hospital and 4 dead at the cardiac surgery hospital in Tajura", the national unity government's health ministry said on Facebook.

It said it would give the number of wounded in a later statement.

Mitiga, a former airbase in eastern Tripoli, has been a civilian airport since the city's main international airport was badly damaged in fighting between rival militias in mid-2014.

The North African country has been wracked by chaos since the 2011 uprising that toppled and killed long-time dictator Moamer Kadhafi, with rival authorities and militias battling for control of its oil riches.

Monday's clashes forced the closure of the airport after a militia in charge of security there said it had been attacked.

"Flights have been suspended because of fighting that broke out this morning," the Facebook page of Mitiga International Airport said.

Al-Radaa, a force loyal to the UN-backed Government of National Accord (GNA) and tasked with keeping the facility secure, said in a statement it had come under attack.

An armed group "has attacked Mitiga international airport... which is home to a prison where more than 2,500 people are detained for various" reasons, Al-Radaa said on Facebook.

'Outlaw attackers apprehended'

Gunmen attacked in a bid "to free" some of their colleagues detained there, it added, without identifying the assailants.

A GNA statement denounced what it called a "premeditated" attack on the airport.

Heavy gunfire could be heard as far away as Tajura 30 kilometres (18 miles) east of Tripoli, AFP reporters said, adding that all roads to Mitiga were closed.

Security forces reporting to the interior ministry said both civilians and military areas of the airport were under control and undamaged, and that "a group of outlaw attackers has been apprehended".

A Libyan pilot said earlier that the airport, the scene of frequent clashes between rival militias, was evacuated when the fighting broke out.

"All the staff and passengers who were at the airport were evacuated," the pilot said, speaking on condition of anonymity.

"We saw tanks in the airport's perimeter," he added.

Since the 2011 revolution, Libya has descended into political and military chaos, with militias who had fought to topple Kadhafi often turning against each other.

Libya has rival administrations, with the authorities in the east not recognising the UN-backed GNA based in the capital.

Al-Radaa is loyal to the GNA and also serves as a police force in Tripoli, as well as tracking suspected jihadists and drug and alcohol smugglers.

The GNA in a statement condemned "the premeditated attack on the perimeter of Mitiga International Airport in Tripoli on Monday... endangering the lives of passengers and the safety of civil aviation".

"This attack was aimed at the release of terrorists belonging to the Islamic State and Al-Qaeda organisations and other groups from the detention centre" run by Al-Radaa, a "deterrent force under the Ministry of the Interior".