US-backed forces launched an attack on Friday to evict Islamic State from its last territorial bastion and seal four years of international efforts to roll back a “caliphate" that redrew the map of the Middle East.

While the fall of Baghouz, an eastern Syrian village on the bank of the Euphrates River, would mark a milestone in the campaign against the jihadists, they are still a threat, using guerrilla tactics and holding some desolate land further west.

Islamic State (IS) followers and fighters fell back to Baghouz as the group was gradually pushed out of its territory, including its twin capitals of Mosul and Raqqa, in 2017.

Thousands of them have poured out of the enclave over the last few weeks, an evacuation that held up the final assault.

The US-backed Syrian Democratic

Forces (SDF) removed the last batch of non-combatants and then moved in at 6 pm (1600 GMT), said Mustafa Bali, head of the SDF media office.

"Nothing remains in Baghouz except for terrorists. The battle ... will not end until the elimination of Daesh and the liberation of the village," he told Reuters, using an Arabic acronym for IS.

"We expect a fierce and heavy battle," he said, adding that initial fighting involved heavy weapons.

The SDF has previously said many of the jihadists left in Baghouz are foreign fighters who travelled from around the world to join IS after its leader declared a modern-day caliphate in 2014 over large swathes of Syria and neighbouring Iraq.

They bore various nationalities including Russian, Indonesian and Azerbaijani. One woman told Reuters that IS militants still inside Baghouz had dug in and were ready to fight to the death.

Some 40,000 people have crossed out of the jihadists' diminishing territory in the last three months as the SDF sought to drive the militants from remaining pockets.

The village was the last populated territory held by the jihadists, who have been steadily driven by an array of enemies from land they once held which at its height was about the size of Britain, by one estimate.

The SDF commander-in-chief said on Thursday that his force would declare victory within a week. He was later contradicted by US President Donald Trump, who said the SDF had retaken 100 percent of the territory once held by IS.