Forces opposed to the UN-backed Libyan government in Tripoli appear to be making a clean sweep through the country’s “oil crescent”, seizing control of oil terminal headquarters and gaining a stranglehold over the export of Libya’s economic lifeblood.

The capture of the oil terminals through the weekend and Monday changes the balance of political forces inside Libya and makes the survival of the UN-backed, Tripoli-based government of national accord (GNA) less likely.

The oil ports were seized by forces under the control of General Khalifa Haftar, who opposes the GNA and supports the rival government in the east of the country. The victory for Haftar is likely to increase his prestige and his negotiating power in the event of Libya being carved up.

The clashes also mean that the possibility of an economic revival driven by oil production and export is further away than ever. Six western nations had issued a joint appeal in August urging that oil facilities be freed from the civil war.

The Libyan national oil corporation, one of the few technocratic bodies left in Libya, had produced a clear plan to revive oil production and exports this year.

Oil production, pipelines and terminals have been at the centre of the civil war since the collapse of the government of Muammar Gaddafi in 2011. Oil production has collapsed from a potential of more than 1.5 million barrels a day to just 200,000.

In a rapid strike on Sunday, the Libyan National Army (LNA) loyal to forces in the east of Libya took control of the neighbouring Sidra and Ras Lanuf oil terminal towns from the central region’s Petroleum Facilities Guards (PFG), headed by Ibrahim Jadhran.

In addition, LNA forces took the south and east gates at Ajdabiya and, overnight on Monday, the headquarters of the Zueitina oil terminal, leaving the PFG with hardly any territory under its control. UK and western forces had been planning to train the PFG to be a more effective fighting force.

The PFG appears to have fled, leaving its weapons, without any attempt to defend the installations. It had previously been accused of demanding bribes to keep the oil installations open.

It is the first time there has been fighting between the LNA and the Tripoli government since the GNA took power in March. Haftar has been demanding a major role in the new government, which has failed to gain support from all the warring factions.

In response to the attacks on the oil installations, the UN-backed government called for the LNA to be defeated. However, there was no immediate sign that any forces, such as the Misrata Brigades, would respond.

The unity government called on all forces loyal to it to “protect and defend” the ports against “flagrant aggression” towards Libyan sovereignty.

It warned overnight that the country was at a “critical juncture”, adding that “the hopes of Libyans for stability in the country have been dashed”.

The head of the rival government in the eastern part of the country 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 from the city of Bayda.

The LNA said the operation had been planned for months and had been achieved largely without any bloodshed.

PFG spokesman Ali al-Hassi at first attempted to deny the LNA had taken Sidra and Ras Lanuf. However, local news media and pictures posted on the internet suggested otherwise.

Ras Lanuf and Sidra include not only the terminals but also a 220,000-barrel-a-day refinery, a petrochemical complex, a military and a civil airport and oil company buildings. They are also home to 25,000 people.Martin Kobler, the UN’s special envoy in Libya, called for an end to the violence in the Libyan oil crescent and respect for the UN-endorsed GNA government as “the sole executive authority in Libya”.

He said UN resolutions contain a clear prohibition on illicit oil exports. “Attacks on the oil terminals further threaten the stability and lead to a greater division of the country. They further restrict the oil exports and add to people’s suffering,” he said.

“I urge all parties to avoid any damage to the oil facilities. Natural resources belong to all Libyans and preserving oil infrastructure is in the national interest.”