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 the jihadists' makeshift encampment, which it overran on Tuesday.

An SDF official who asked not to be named said warplanes from the US-led coalition resumed airstrikes 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 "operation to complete the liberation of Baghouz is ongoing," the US-led coalition said.

"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 made up 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 on Thursday said a declaration of victory will be made only after they have completed flushing out the last tunnels and hideouts.

According to the Kurd-led force, several jihadists are still inside the last pocket.

The Britain-based Syrian Observatory for Human Rights on Friday confirmed IS holdouts were still hiding in underground tunnels and caves in Baghouz. But could give no figures for how many may still remain.

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

They include 37,000 civilians, 5,000 jihadists and around 24,000 of their relatives.

Thousands of people, who have streamed out of IS's last stronghold, have now crowed into cramped camps and prisons run by the Kurds further north.

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

At least 138 people, mostly children, have died en route to Al Hol or shortly after arriving at the camp since December, 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 final 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 an IS video released on the group'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 aimed at deposing longtime leader President Bashar al-Assad has killed more than 370,000 people and displaced millions since it erupted following the repression of anti-government protests in 2011.