Fighters from the Islamic State of Iraq and the Levant (ISIL) group have clashed with a force guarding Libya's Es Sider oil export terminal, witnesses say.
ISIL also claimed on Monday to have taken over the nearby town of Ben Jawad, which sits between the cities of Sirte and Benghazi.
The group said in an online statement it had also set off a suicide car bomb during the clashes, causing casualties.
There was no one from Libya's authorities immediately available to comment on the the capture of Ben Jawad, Reuters news agency said.
Ali al-Hassi, a spokesman for the forces that control the majority of Libya's oil fields, says six of their fighters were killed in Monday's attacks, along with five ISIL fighters in the coastal port town of Sidra, Associated Press news agency reported.
Es Sider and the nearby Ras Lanuf oil ports, on the Mediterranean coast, have been closed for more than a year amid fighting between rival factions for control of the North African state and its energy reserves.
ISIL controls the city of Sirte and has attacked several oilfields in the south of Libya, though it has so far not taken control of any oil installations as it has done in Syria.
Libya has been split between rival governments, one based in Tripoli and the other in the east of the country, creating a security vacuum that armed groups have exploited.
Es Sider is protected by Ibrahim al-Jathran's Petrol Facilities Guard, an armed faction which has backed the internationally recognised government in the east, but it is in conflict with other forces supporting that government.
The UN has been trying to win support for a deal brokered in Morocco last month to create a national unity government for Libya.
Separately on Monday, a military plane that was targeting armed groups in the eastern Libyan city of Benghazi was shot down, though the pilot ejected, senior army commander Fadel al-Hassi said.
Military forces allied to the internationally recognised government based in the east have been battling armed groups based in the city for months.