Tripoli (AFP) - Libya's internationally backed government urged its forces Sunday to act after two oil terminals fell to rival troops, raising fears of further violence in a country already gripped by turmoil.

The call came after forces loyal to the unrecognised eastern authority seized two key export facilities.

The Tripoli-based Government of National Accord called on all forces loyal to it to "protect and defend" the ports against what it called "flagrant aggression" of Libyan sovereignty.

The two Mediterranean ports are in Libya's "oil crescent", and seen as a vital source of income for the GNA which is struggling to assert its authority.

Previously controlled by guards allied to the GNA, the ports of Al-Sidra and Ras Lanuf were seized by forces loyal to General Khalifa Haftar.

One of the most powerful military figures in Libya, Haftar is allied with a rival authority to the GNA based in the east.

Ahmad Mesmari, a spokesman for Haftar's forces, told a press conference that another oil terminal at Zuwaytina was not yet under their control.

A colonel with pro-GNA forces, allied to guards defending the facilities, confirmed that Haftar's fighters were in control of Al-Sidra and Ras Lanuf.

Hatem El-Ouraybi, a spokesman for the eastern authority, told AFP the attack was aimed at "regaining full control of the oil crescent".

"The government calls on all the people of the oil crescent area -- including those who were in the oil installations guards -- to join the army or return to their homes," he said.

But the GNA on Facebook urged all "military forces" loyal to the unity government "to protect and defend the oil installations and terminals and to carry out their military and national duties bravely and without hesitation".

The GNA also called on Haftar's forces to "immediately withdraw from all the sites they have attacked".

Ras Lanuf and Al-Sidra are together capable of handling 700,000 barrels of oil per day but had been closed for months after jihadist attacks.

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- 'Blow to oil industry' -

In late July, the oil installation guards announced the reopening of the two ports after an agreement with the unity government to resume oil exports.

They had been closed following attacks in January by the Islamic State group, who took advantage of turmoil after the 2011 uprising to gain a foothold in the country.

In recent weeks, pro-GNA forces backed by US air strikes have been pressing a months-long campaign to expel the last IS jihadists from what was their North African stronghold.

IS took the city of Sirte in June last year, sparking fears they would use the city west of the oil crescent as a launchpad for attacks in Europe.

Haftar's forces have since 2014 been fighting jihadists in the second city of Benghazi northeast of the oil-rich crescent.

UN Libya envoy Martin Kobler said on Twitter he was worried about the fighting.

"Oil belongs to ALL Libyans," he tweeted. "Conflicts can only be solved through dialogue, not violence. Urge all parties to sit 2gether."

Mattia Toaldo, a Libya expert with the European Council on Foreign Relations, said the seizure of the terminals could deal a fatal blow to Libya's oil sector.

"Unless there is a very quick mediation, this could lead to conflict and a final blow to Libya's oil industry," Toaldo said.

Oil is Libya's main natural resource with reserves estimated at 48 billion barrels, the largest in Africa.

But since 2010 the country's production has plummeted from 1.5 million bpd to just 200,000 bpd.

- 'Undermining' GNA -

Libya's oil sector is managed by the National Oil Company which is split into two rival branches, one allied to the GNA and the other to the authority in the east.

Once pro-GNA forces expel IS from Sirte and there is no buffer between them and Haftar's army, either side "could easily clash again", Toaldo said.

The pro-GNA military command is based in the city of Misrata.

The attack on the ports aimed to "take advantage of Misrata's exhaustion after Sirte" and "undermine any negotiation on the future of the GNA", Toaldo said.

Libya has been in chaos since the 2011 revolt that toppled and killed longtime dictator Moamer Kadhafi, with rival authorities and militias vying to control the country.

A UN-brokered deal in December led to the unity government starting to work in Tripoli, but it has struggled to assert its authority.

The parliament in the east last month voted no confidence in the GNA in a setback to efforts to end the political chaos.