Kurdish-led forces in eastern Syria prepared on Friday for a push on the last remaining speck of the Islamic State group's “caliphate” where diehard militants and their families are holed up.

US President Donald Trump predicted that the once-sprawling proto-state's official death could be proclaimed as early as next week but operations have been paused for days on the main front line.

Four years ago, IS controlled territory the size of Britain and administered millions of people, but the US coalition fighting the group said 99.5 per cent of it has been clawed back in successive offensives.

Its deputy commander, Major General Christopher Ghika, described the size of the last IS pocket as “now less than one percent of the original caliphate.”

The coalition has been training and providing air support to the Syrian Democratic Forces, which launched an offensive on the last pocket of militant territory in September 2018.

Two months later, they took Hajin, which was the last town of note under IS control and left the militants fighting over a few scattered hamlets in the Euphrates Valley.

Thousands of suspected IS fighters have attempted to blend in with civilians fleeing the militants' last bastion, including a large number of foreigners.

“They are attempting to escape through intermixing with the innocent women and children attempting to flee the fighting,” Ghika said on Thursday.

'Next week'

The SDF have set up screening centres to process the droves of haggard people streaming out of IS-held territory, often famished and covered in dust.

British, French, US and other forces are actively looking for wanted IS operatives among those fleeing the combat zone with civilians.

After weeks of advancing steadily, the SDF halted their ground assault on IS's tiny remaining enclave.

Know more: 20,000-30,000 IS fighters left in Iraq, Syria: UN

“There hasn't been any big advance or change on our side over the past five days,” an SDF spokesman told AFP on Friday.

“Currently, the SDF is advancing very cautiously to ensure the safety of civilians that IS is using as human shields.”

The Kurds, who have de facto autonomy in northeastern Syria, are also engaged in behind-the-scenes diplomatic talks over the fate of the region.

Trump said in December that he had ordered a complete troop withdrawal from Syria, a shock announcement that left the Kurds scrambling for new allies.

“We are seeking a political solution in Syria, which requires an agreement with the government in Damascus. We have chosen a political agreement with Damascus because we don't want secession from Syria,” senior Kurdish official Badran Jia Kurdi told AFP.

Speaking at the State Department on Wednesday, Trump said that US-led troops and their Kurdish allies should formally announce the end of the “caliphate” some time next week.

“Remnants — that's all they have, remnants — but remnants can be very dangerous,” Trump said.

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While IS will soon no longer have fixed positions anywhere in Iraq or Syria, its surviving fighters have reverted to guerrilla warfare and remain a potent force.

The militants maintain sleeper cells along the border with Iraq, as well as in cities they once ruled, and have carried out periodic hit-and-run attacks.

The SDF arrested 63 suspected militants in IS's former Syrian capital Raqa on Thursday during an operation against sleeper cells, it said in a statement.

At least 48 suspected IS members were among them, according to the Syrian Observatory for Human Rights.

The Observatory says IS sleeper cells are suspected of assassinating at least 50 civilians and 135 SDF fighters in Kurdish-held territory since August.

No 'transparency'

More than 37,000 people, mostly wives and children of militant fighters, have fled IS territory since the SDF, backed by the US-led coalition, intensified its offensive in December, according to the Observatory.

The Britain-based war monitor has said that figure includes some 3,200 suspected militants.

Kurdish authorities say they have in their custody hundreds of foreign male IS members.

Suspected militants captured by the SDF are usually eager to be sent home, something the Kurdish administration also wants.

Governments in the miltants' countries of origin are often reluctant, although France — which has one of the largest contingents — recently said it would consider limited repatriations.

Human Rights Watch has warned that any transfers of suspected foreign militants and their relatives out of Syria should be transparent.

“As we speak, there may already be transfers happening. There's been a total lack of transparency, and bad things happen in the dark,” HRW's counterterrorism director, Nadim Houry, told AFP.