Forces loyal to a Libyan general on Sunday said they had seized two key oil export terminals as a UN-backed unity government struggles to assert its authority over the oil-rich country.

If confirmed, the seizure of the ports would deal a major blow to the Tripoli-based Government of National Accord (GNA), which had hoped to rely on oil exports for its revenues.

General Khalifa Haftar's forces took control of the Al-Sidra and Ras Lanuf ports, the LANA news agency loyal to a rival authority reported a spokesman as saying.

"Clashes are now ongoing near the Zuwaytina port" further east in the same oil-rich area seen as key to the country's economy, Colonel Ahmad Mesmari said.

Haftar, one of the most powerful military figures in Libya, has refused to endorse the GNA and remains loyal to the authority based in the east of the country.

UN Libya envoy Martin Kobler on Twitter said he was worried about the reported fighting in Libya's so-called "oil crescent".

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

Haftar's forces took the Ras Lanuf and Al-Sidra terminals -- together capable of handling 700,000 barrels of oil per day -- from installation guards loyal to the GNA.

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.

The terminals had been closed for months following attacks in January by the jihadist 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 have been pressing to expel the last IS jihadists from what was their North African stronghold of Sirte.

The jihadists took over the city -- some 180 kilometres (110 miles) to the east of the oil terminals -- in June last year.

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

A UN-brokered deal in December led to the GNA starting to work in Tripoli, but it has since struggled to assert its authority over the country.

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

But the country's production has plummeted from 1.5 million bpd to just 300,000 bpd since 2010.

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.