GAZIANTEP, Turkey — A Syrian rebel brigade defected to the Islamic State this week, a sign that the extremist group continues to build strength after seizing vast territories in western Iraq and eastern Syria, anti-government activists said Tuesday.

The 1,000-strong Dawud Brigade, which had been based in Sarmin, a town in Syria’s Idlib province, arrived Sunday in Raqqa, a city in northeast Syria that the Islamic State has made its main headquarters for more than a year.

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. In order to cross the lines of pro-Western rebels who are fighting the Islamic State, 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 the Islamic State, and the impact of its departure from an anti-government umbrella group, the Sham Army, wasn’t immediately clear.