Islamic State militants on Monday clashed with a force guarding Libya's Es Sider oil port, and fighting continued near the major export terminal, witnesses and troops said.

Es Sider and nearby Ras Lanuf oil ports have been closed for more than a year after fighting between rival factions for control of the country, where Islamic State has gained ground in the turmoil since the 2011 fall of Muammar Gaddafi.

Es Sider is protected by Ibrahim al-Jathran's Petrol Facilities Guard, a federalist armed faction.

Islamic State controls the city of Sirte and has slowly expanded its presence in the North Africa state. Its fighters have attacked several oilfields in the south of Libya, but it has so far not managed to control any oil installations as it has done in Syria.