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. The big question was whether other groups or individuals would follow suit.

The Islamic State’s leader, Abu Bakr al-Baghdadi, has called for Muslims throughout the world to travel to the Islamic caliphate the group has established in the areas of Iraq and Syria it controls.

One Syrian journalist put down Dawud’s departure to threats made by other rebel groups that feared the brigade as a potential fifth column that could wreak havoc in an area from which the extremists were thought to have been expelled in January. The Sham Army claimed that it had expelled Dawud.

The journalist, Ammar Abu Shahin, said the expulsion came after Islamic State advances in Syria’s Deir el-Zour and Aleppo provinces prompted anti-Islamic State rebels in Idlib and Hama provinces to look for sleeper cells.

“Frankly speaking, people in the countryside of Idlib are in a real panic about the advancement of the Islamic State,” he said.

He said the Islamic Front, an umbrella group of Islamist fighting forces, had arrested and executed eight Islamic State agents, and that this drove Hassan Abboud, the brigade’s leader, to head to Raqqa. This “suits him, as the Islamic State is in charge,” he said.

Dawud had long been under suspicion by other groups. It had fought on the side of the Islamic State against the pro-Western Free Syrian Army late last year and well into January, but after FSA commander Jamal Marouf routed the Islamic State from most of its bases in Idlib in January and threatened Dawud, it quit the alliance. After declaring that it would direct its weapons only against the regime, Dawud joined the Sham Army in February.

But the brigade continued to straddle the fence, said Muhiddin Abdul Razzak, an anti-government activist in Sarmin. He charged that Dawud had provided a haven for Syrians and foreign volunteers who’d fled the besieged Islamic State forces but intended to rejoin if the opportunity arose.

He called Dawud’s decision to join the Sham Army “only a change of uniforms” and said other Islamic State supporters might now declare themselves and also head to Raqqa.

“We’d really love to get rid of that rubbish,” he said.

Dawud’s Abboud couldn’t be reached for comment.

Al Hamadee is a McClatchy special correspondent.