BENGHAZI, Libya (REUTERS) - Islamic State in Iraq and Syria (ISIS) militants have seized another town in Libya, the group and a military source said on Friday, expanding the territory they control in the strife-torn country.

The militant group, which controls large parts of Iraq and Syria, has exploited a security vacuum in Libya as two rival governments struggle for power, four years after the ousting of Muammar Gaddafi.

ISIS took over the city of Sirte on Libya's central Mediterranean coast in stages this year, occupying government buildings and taking the city's airport last month.

The group has now also taken over the town of Harwa to the east of Sirte, according to a statement posted on social media. A military source confirmed militants were controlling Harwa, adding they had taken over government buildings.

ISIS has claimed responsibility for the killings of dozens of Egyptian and Ethiopian Christians, the storming of a luxury hotel in Tripoli and attacks on oilfields, as well as suicide bombings in several cities.

The group has a strong presence in the eastern city of Derna and has carried out suicide attacks in Benghazi, the main eastern city.

The government of Premier Abdullah al-Thinni has been based in the east since losing the capital Tripoli in August to a rival administration.