Fighting has broken out over the future of Libya’s largest oil field, as forces loyal to the UN-recognised Tripoli-based government battle Libyan National Army (LNA) forces led by Field Marshal Khalifa Haftar, the leading figure in fractured Libya’s east.

Al-Sharara field, 560 miles south of Tripoli, is capable of producing 315,000 barrels of crude a day – about a third of Libya’s total current output. But it has been closed by the Libyan National Oil Corporation (NOC) since December when the installation was seized by local tribes demanding the Tripoli government did more to lift the area out of poverty.

The fighting has the potential to disrupt the UN’s long prepared plans to convene a national conference, possibly next month, that is supposed to lead to either parliamentary or presidential elections and a new constitution. No date or venue for the conference has been set by the UN, which is still trying to win an agreement on those attending the meeting and the broad agenda.

Haftar forces, already in control of large tracts of Libyan oil including in the “oil crescent” in the north, moved south last month in what was billed as an operation to push out terrorists and militias.

An LNA spokesman, Lt Gen Ali Suleiman Muhammad claimed on Wednesday that Haftar’s forces had seized al-Shara oil fields largely without a fight, in conjunction with the forces that had previously controlled the field.

This was later contradicted by other local reports that suggested five people had been killed and 16 injured in the fighting.

map Guardian graphic | Source: Political Geography Now, Petroleum Economist | Note: areas of control as of July 2018

The UN-backed Government of National Accord denounced Haftar’s move and said forces loyal to the GNA had travelled south determined to protect the fields from Haftar.

If Haftar can capture the southern oil field, he would have a lock on the bulk of Libyan oil production, placing him in a stronger position in any UN-supervised elections due this year. The oil crescent contains the major oil exporting terminals.

It took weeks of intense international diplomatic pressure last June to persuade Haftar to agree that the revenues from the crescent were sent on to the Tripoli-based NOC, and not to a smaller rival oil company based in the east. Markets were deprived of some 800,000 barrels a day and Libya lost $930m (£718m) in sales as a consequence.

The corporation has been one of the few Libyan institutions that has operated effectively as a national non-partisan force, and has been providing much-needed cash to keep the Libyan budget from falling deeper into debt. The NOC wants to drive oil production from 1m barrels a day to as high as 2.1m by 2021, but the conflict over al-Sharara oil field puts that objective in doubt.

Haftar’s push south appears to have multiple motives, including his claim to be rooting out terrorists and militia from remote areas that have long acted as a funnel for migrants coming up through Africa. He has also claimed Chadian forces trying to dislodge their country’s president, Idriss Déby, have been operating in the area.

Haftar’s southward move was helped on Sunday by French air forces. Paris admitted at the weekend its warplanes had bombed a column of 40 rebel pickup trucks after they crossed into northern Chad from Libya – insurgents the LNA claimed were fleeing its offensive.

A rebel group opposed to Déby said it had been the target of the strikes.

France’s foreign ministry said on Tuesday that Haftar’s operation had “eliminated terrorist targets” and was a way to “durably hinder the activities of human traffickers”.

Southern Libya is one of the most lawless parts of a country that has been fractured by a six-year civil war. The region is also the scene of a struggle between Libya’s minority Tubu community and Arab tribes, particularly over the control of cross-border smuggling routes.

Libya’s Tubu, part of a larger cross-border ethnic group, have long complained their rights are not recognised in a largely Arab country. Some of its members accuse the LNA, which counts Tubu fighters among its ranks, of directing rival Arab tribes to disrupt their villages and towns.

In a bid to bring greater order to the oil industry, the NOC chair Mustafa Sanallah would like to establish a truly national oil protection guard to protect the sites from repeated capture by militias. Youssef Kalkouri, a Tubu lawmaker in the eastern Haftar-backed administration, told agencies his community categorically opposes Arab tribal forces entering their cities.