Reuters reports that the Islamist group Operation Dawn has taken control of Libya's largest oilfield, El Sharara, which is operated by Spanish oil major Repsol.
The Tripoli-based rival administration has also appointed its own oil minister.
The National Oil Corporation (NOC) said El Sharara would reopen by Wednesday.
In another sign of shifting power, Sudanese foreign minister Ali Karti flew to Tripoli to meet Omar al-Hassi, prime minister of the rival government. A Turkish envoy had already met Hassi, who has so-far been shunned by Western powers.
(Picture: Repsol operations at Sharara)