Gaziantep, Turkey: A Syrian rebel brigade has defected to the Islamic State of Iraq and the Levant, a sign that the extremist group continues to build strength after seizing vast territories in western Iraq and eastern Syria, anti-government activists said.

The 1000-strong Dawud Brigade, which had been based in Sarmin, a town in Syria's Idlib province, arrived on Sunday in Raqqa, a city in north-east Syria that ISIL, which has renamed itself the Islamic State, has made its main headquarters for more than a year.

A Free Syrian Army fighter at the Handarat camp, Aleppo. Credit:Reuters/Hosam Katan

The defecting rebels moved in a convoy of more than 100 vehicles, including 10 tanks that had been seized from the Syrian army, the activists said. To cross the lines of pro-Western rebels who are fighting ISIL, the defecting rebels said they were heading to Aleppo to confront government forces now attempting to lay siege to rebel-held parts of Syria's biggest city.

Dawud, with mostly Islamists in its ranks, has a complex history of relations with ISIL, and the impact of its departure from an anti-government umbrella group, the Sham Army, wasn't immediately clear. The big question was whether other groups or individuals would follow suit.