Friday's bombardment ended two days of relative calm on the front line in the remote village of Baghouz near the Iraqi border.

The Kurdish-led Syrian Democratic Forces had paused its advance while it combed a makeshift jihadist encampment, which it overran on Tuesday.

An SDF official who asked not to be named said warplanes of the US-led coalition resumed strikes on suspected jihadist positions before dawn on Friday.

Top SDF commander Jia Furat said his forces were engaging with the jihadists on several fronts while the coalition warplanes provided air support.

The US-led coalition said the "operation to complete the liberation of Baghouz is ongoing".

"It remains a hard fight, and Daesh is showing that they intend to keep fighting for as long as possible," it said, using an Arabic acronym for IS.

The SDF launched what it called its "final assault" against the jihadists' last redoubt in the village of Baghouz on February 9.

Finally on Tuesday, they cornered diehard fighters into a few acres of farmland along the Euphrates River, after forcing them out of their rag-tag encampment of tents and battered vehicles.

- Tunnels and caves -

The six-month-old operation to wipe out the last vestige of IS's once-sprawling proto-state is close to reaching its inevitable outcome, but the SDF has said a declaration of victory will be made only after they have completed flushing out the last tunnels and hideouts.

According to SDF spokesman Kino Gabriel, hundreds of IS fighters, including some women, still remain on the outskirts of the jihadist encampment.

They are hiding along the bank of the Euphrates River as well as at the base of a hill overlooking Baghouz, he told AFP.

"In around one or two days, we will conclude military operations if there are no surprise developments," he said.

The Britain-based Syrian Observatory for Human Rights said IS holdouts were hiding in underground tunnels and caves in Baghouz.

SDF official Jiaker Amed said several jihadists want to surrender but are being prevented from doing so by other fighters.

"We are trying our best to wrap up the operation without fighting, but some of them are refusing to surrender," he said.

More than 66,000 people, mostly civilians, have quit the last IS redoubt since January 9, according to the SDF.

They compromise 5,000 jihadist fighters and 24,000 of their relatives as well as 37,000 other civilians.

The thousands who have streamed out have been housed in cramped camps and prisons run by Kurdish forces further north.

On Wednesday night, around 2,000 women and children from Baghouz arrived at the largest camp, Al-Hol, which is struggling to cope with the influx of tens of thousands of people, many in poor health.

Since December, at least 138 people, mostly children, have died en route to Al-Hol or shortly after arrival, according to the International Rescue Committee.

- Calls for more attacks -

IS declared a "caliphate" in June 2014 after seizing a vast swathe of territory larger than Britain straddling Iraq and Syria.

The loss of the Baghouz enclave would signal the demise of the "caliphate" in Syria, after its defeat in Iraq in 2017.

But IS has already begun its transformation into a guerilla organisation, and still carries out deadly hit-and-run attacks from desert or mountain hideouts.

In a video released on IS's social media channels on Thursday, jihadists vowed to continue to carry out attacks.

"To those who think our caliphate has ended, we say not only has it not ended, but it is here to stay," said one fighter.

He urged IS supporters to conduct attacks in the West against the enemies of the "caliphate".

The war in Syria has killed more than 370,000 people and displaced millions since it erupted following the repression of anti-government protests in 2011.