Eastern Libyan military forces have full control of Libya's biggest oilfield, El Sharara, a spokesman said, in a challenge to the internationally recognised government in Tripoli, Reuters reports.
There was no immediate confirmation or comment from state oil firm NOC which operates with foreign partners the 315,000 barrels-per-day (bpd) field deep in Libya's southern desert.
LNA (the Libyan National Army) based in eastern Libya began an offensive in the south last month to fight militants and secure oilfields.
Last Wednesday, the LNA reached a remote pumping station of El Sharara, but had until now not moved on the main field, which a force of state guards and tribesmen seized in December demanding funds. Production stopped then.
"Peacefully with no resistance LNA has full control over Sharara oilfield with all its facilities," the LNA spokesman said on Twitter, adding the facility was being secured.
Pictures posted online showed jubilant soldiers standing in front of an El Sharara signpost. Reuters was unable to reach workers at the field for further information.
Hours earlier, the LNA's chief of staff, Abdel-Razeq Nathouri, told Reuters the soldiers were negotiating access to the field.
The internationally recognised government in Tripoli had tried sending its own commander, Ali Kennah, to secure the field. El Sharara had formally belonged to the Tripoli administration which in reality had been unable to exercise control.
Kennah had flown on the weekend to the nearby El Feel oilfield and later made it to El Sharara, but failed to take control of it, oil workers have said.
Oil production in Libya, a member of the Organization of the Petroleum Exporting Countries, has been disrupted since conflict broke out in 2011, with protesters and armed groups often targeting oilfields and energy infrastructure.
National production stands at under 1 million bpd, well below pre-2011 capacity of 1.6 million bpd.