The black flag of ISIS flies over government buildings. Police cars carry the group's insignia. The local football stadium is used for public executions. A town in Syria or Iraq? No. A city on the coast of the Mediterranean, in Libya.

Fighters loyal to the Islamic State in Iraq and Syria are now in complete control of the city of Derna, population of about 100,000, not far from the Egyptian border and just about 200 miles from the southern shores of the European Union.

The fighters are taking advantage of political chaos to rapidly expand their presence westwards along the coast, Libyan sources tell CNN.

The sources say the Derna branch of ISIS counts 800 fighters and operates half a dozen camps on the outskirts of the town, as well as larger facilities in the nearby Green Mountains, where fighters from across North Africa are being trained.

It has been bolstered by the return to Libya from Syria and Iraq of up to 300 Libyan jihadists who were part of ISIS' al Battar Brigade -- deployed at first in Deir Ezzor in Syria and then Mosul in Iraq. These fighters supported the Shura Council for the Youth of Islam in Derna, a pro-ISIS faction.

The council had been competing for superiority with another militant group, the Abu Salem Brigade, some of whose fighters' loyalties lay with al Qaeda, according to Noman Benotman, a former Libyan jihadist now involved in counter-terrorism for the Quilliam Foundation....