DEIR EZZOR PROVINCE, Syria (REUTERS) - US-backed Syrian fighters launched an operation on Friday (March 1) to clear the last remaining pocket of Islamic State in Iraq and Syria (ISIS) fighters from the besieged eastern Syrian village of Baghouz after weeks of delays caused by the evacuation of thousands of civilians.

The Syrian Democratic Forces (SDF) moved on the enclave, a tiny area on the eastern bank of the Euphrates River, at 6pm (midnight on Saturday, Singapore time) after the last batch of civilians were removed, said Mustafa Bali, the 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 ISIS.

Bali said the initial fighting involved heavy weapons. Asked how long the battle would last, he said: "We expect a fierce and heavy battle."

The ISIS enclave at Baghouz, a tiny pocket on the eastern bank of the Euphrates River, is the last populated territory held by the militants, who have been steadily driven by an array of enemies from swathes of land they once held.

Though the fall of Baghouz will mark a milestone in the campaign against ISIS, the group continues to be seen as a security threat, using guerrilla tactics and holding some desolate territory in a remote area west of the Euphrates River.

The SDF commander-in-chief said on Thursday that his force would declare victory over the extremists in one week.

The SDF has been poised for several weeks to wipe out the last vestige of ISIS' territorial rule in Baghuz, but the operation has been held up by efforts to evacuate thousands of civilians.

US President Donald Trump said on Thursday that the SDF had retaken 100 per cent of the territory once held by ISIS, but Bali said ISIS militants were still holed up there and had not surrendered.

Some 40,000 people have crossed out of the extremists' diminishing territory in the last three months as the US-backed SDF sought to drive it from the remaining territory.

The numbers of evacuees pouring out of Baghuz have surpassed initial estimates of how many were inside.

An SDF commander told Reuters on Thursday that many of the people coming out of Baghuz had been underground in caves and tunnels.

A spokesman for the US-led international coalition which supports the SDF said the Kurdish-led Syrian group had adopted a"slow and deliberate" approach to Baghuz.

"They are dealing with multiple dilemmas and trying to stabilise the area," Colonel Sean Ryan said.

The United States has about 2,000 troops in Syria, mainly to support the SDF in fighting against ISIS.

Mr Trump announced in December he would withdraw all of them because ISIS had already been defeated, a decision that shocked allies and top aides and prompted Defence Secretary James Mattis to quit.

Earlier this month, the White House partially reversed itself and said around 200 US troops would stay.