Roman theatre of Sabratha
The ancient Roman theatre of Sabratha, 90 km west of the Libyan capital Tripoli, September 29, 2011.Getty

A rapid advance by Islamic State (Isis) forces into Sabratha in Libya, just 50 miles west of the country's capital Tripoli, has raised fears over the destruction of ancient Roman ruins in the town, a Unesco heritage site.

A convoy of roughly 30 technical vehicles entered down town Sabratha flying the IS (Daesh)'s black flag, in retaliation for the arrest of two of its members, a Tunisian and a Libyan, local media reported. The pair, who had recently returned from fighting in Syria were arrested by local security forces because of the Libyan's apparent affiliation with Isis.

Sabratha, which has been a hotspot of Islamist activity since the Libyan revolution in 2011, is also the location of a startlingly well-preserved 3rd century Roman theatre, as well as a series of mosaics.

Isis, as it seeks to impose its dominance over Libya from its Sirte stronghold to the east, may look to make a show of strength through the kind of ostentatious iconoclasm exhibited by Daesh in Syria at Palmyra. The militant group destroyed the centuries old Temple of Baalshamin at the ancient ruins as well as a series of columns and other Roman temples.

Whether IS (Daesh) has retained its position in Sabratha remains unclear. Initial reports indicated the group had set up checkpoints around the town before entering but they may have subsequently withdrawn.

Sabratha local council, which is aligned with the Libya Dawn militias in control of Tripoli, has said it is reviewing its procedures to ensure that proper security measures are put in place in the wake of the incursion, indicating that it has reasserted its control.

The relative ease with which IS was able to advance into the western Libyan town, with virtually no opposition from local militias, is an indication of support for the extremists there and the fluidity of allegiance to different brigades for many fighters in the region.

Prior to the advent of Isis in Libya just over a year ago Sabratha played a key role in moving fighters to Syria, many of whom subsequently returned. Ansar al-Sharia, which was the pre-eminent extremist group prior to the arrival of IS, set up training camps at farms not far from Sabratha.

Isis and Ansar al-Sharia, an al-Qaeda affiliate, fight alongside each other in different regions of Libya including in Benghazi and Ajdabia to the east.