Libya's UN-backed unity government lost control on Monday of a third oil port seized by rival forces, raising fears of a major outbreak of fresh violence in the chaos-ridden country.

The loss comes after fighters backing a rival administration in east Libya seized two other terminals from guards loyal to the Tripoli-based Government of National Accord (GNA) on Sunday.

All three Mediterranean ports are in Libya's "oil crescent", an area seen as a vital source of income for the GNA which has struggled to assert its authority across the country.

"Our armed forces were able to take control of Zuwaytina port and secure it completely," a spokesman for the fighters who took the terminals said.

The LANA news agency loyal to the eastern administration reported a military source as saying: "The armed forces are now concentrated at the port and have secured it after expelling outlaw militia from it."

The forces are commanded by controversial General Khalifa Haftar, who has refused to back the GNA and supports the parallel authority based in eastern Libya near the border with Egypt.

On Sunday, they took the Al-Sidra and Ras Lanuf ports before attacking Zuwaytina to the east.

It is the first time that Haftar's forces and fighters loyal to the GNA have clashed directly since the unity government started working in the capital in March.

The UN special representative to Libya, Martin Kobler, urged Haftar's forces "to immediately stop fighting and refrain from further military escalation".

"I call for the respect of UN Security Council Resolution 2259 which recognises the Government of National Accord... as the sole executive authority in Libya," he said in a statement.

Kobler said oil installations must remain under the authority of the presidential council and stressed that Resolution 2259 "contains a clear prohibition on illicit oil exports".

Meanwhile the United States and its major European allies condemned the offensives.

"The governments of France, Germany, Italy, Spain, the United Kingdom and the United States condemn this weekend's attacks on Zuwaytina, Ras Lanuf, Es Sider and Brega oil terminals in Libya," they said, in a statement.

- 'Critical juncture' -

The unity government, for its part, called on all forces loyal to it to "protect and defend" the ports, while the head of the rival government in the eastern city of Bayda said it would work on reopening the ports as soon as possible.

"We will work on the oil ports resuming work as soon as possible so as to guarantee all Libyans a decent life," Abdullah al-Thani said.

Haftar's forces said they would hand over management of the ports to the National Oil Corporation (NOC) to resume oil exports.

The NOC is split into two rival branches, however, one allied to the GNA and the other to the administration that Haftar supports.

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.

The seizure of the ports deals a heavy blow to the unity government, depriving it of a key source of income.

In late July, the GNA agreed to resume oil exports out of Ras Lanuf and Al-Sidra -- which together have a capacity of 700,000 bpd -- after a months-long closure following jihadist attacks.

They shut after attacks in January by the Islamic State (IS) group, which has gained a foothold in the country.

- Rival authorities, militia -

Libya has been in chaos since the 2011 uprising that toppled and killed longtime dictator Moamer Kadhafi, with rival authorities and militia vying for control.

The port of Brega -- between Ras Lanuf and Zuwaytina -- remains in the hands of the oil installation guards.

Haftar's assault on the ports came as pro-GNA forces, including oil installation guards, have been battling for months to oust IS from the coastal city of Sirte.

The US Africa Command, meanwhile, said American warplanes carried out four air strikes in Sirte on Sunday in support of GNA forces, targeting IS "fighting positions".

The latest raids brought to 143 the total number of air strikes in support of the loyalist fighters since they began on August 1, said a statement issued Monday.

Italian Foreign Minister Paolo Gentiloni said Monday his country would establish a military field hospital in the Libyan city of Misrata, to where GNA casualties from Sirte are evacuated, and deploy troops to ensure security.

Italy's La Repubblica newspaper said the operation would involve 100 medical staff and 200 paratroopers.

Source: AFP