BAGHOUZ, Syria (Reuters) - U.S.-backed Syrian forces launched an assault against the final Islamic State enclave in eastern Syria on Sunday, aiming to wipe out the last vestige of its self-declared “caliphate” that once spanned a third of Iraq and Syria.

While the Baghouz enclave represents the last shred of populated land held by the jihadists, the group is still widely seen as a big security threat operating in remote territory elsewhere and able to launch guerrilla attacks.

The U.S.-backed Syrian Democratic Forces (SDF) has been poised to advance into the enclave for weeks, but has repeatedly held back to allow for the evacuation of civilians, many of them wives and children of Islamic State fighters.

Mustafa Bali, head of the SDF media office, said no further civilians had emerged from the enclave at the Iraqi border since Saturday and the SDF had not observed any more civilians in the area, prompting the decision to attack.

“The military operations have started. Our forces are now clashing with the terrorists and the attack started,” he told Reuters.

Bali said more than 4,000 IS militants had surrendered in the last month, among the tens of thousands of people who have streamed out of Baghouz - a collection of hamlets surrounded by farmland on the eastern bank of the Euphrates River.

But the SDF has also said the most hardened foreign jihadists are still holed up inside, ready to fight to the end.

In a post on Twitter, Bali said the attack had started at 6 p.m. (1600 GMT). Air strikes had targeted Islamic State weapons stores. “Direct and fierce” clashes were underway, he added.

Islamic State has been driven from the territory it once held in Syria and Iraq by an array of enemies, including forces backed by Russia, Iran and Turkey in addition to the United States.

It suffered its major military defeats in 2017 with the loss of the Syrian city of Raqqa and Iraq’s Mosul.

A former IS fighter who surrendered to the SDF two months ago told Reuters in an interview on Saturday that the group had sent hundreds of men out of its diminishing territory to establish sleeper cells.

The SDF, spearheaded by the Kurdish YPG militia, has been the main U.S. partner in Syria and has steadily driven the group down the eastern bank of the Euphrates River, forcing its fighters and followers to fall back to Baghouz.

Earlier on Sunday, a Reuters journalist in SDF-held ground observed Islamic State fighters moving around in the part of Baghouz still under their control.

SDF fighters advanced into a makeshift tented settlement abandoned by the group, finding a number of rifles and ammunition left behind.