Holdout extremist militants scurried along the reedy banks of the Euphrates in an increasingly desperate defence on Tuesday of the last scrap of their 'caliphate' in eastern Syria.

Advancing Kurdish-led forces forced fighters from the militant Islamic State (IS) group out of the main encampment where they had been confined in recent days.

The move brought a months-old operation to wipe out the last vestige of IS's once-sprawling proto-state closer to its inevitable outcome, but the Syrian Democratic Forces (SDF) stopped short of declaring victory.

Some of the militants now fighting for a few fields in a bend of the river seemed unwilling to surrender and a senior IS leader even issued a message calling for attacks.

"This is not a victory announcement, but a significant progress in the fight against Daesh," SDF spokesman Mustefa Bali said.

A few days ago, the last denizens of the IS 'caliphate' were crammed into a chaotic encampment on the edge of Baghouz, a jumble of mangled vehicles, hastily-dug berms and makeshift tents.

Thousands of people — fighters, their relatives and other civilians — fled the death trap in recent weeks and turned themselves in after scaling a nearby hill to a position held by the SDF.

The Kurdish-led force, which also includes fighters from local Arab tribes, moved in on Tuesday and took up positions in the deserted encampment, leaving holdout jihadists cornered.

Syrian regime and allied forces are deployed on the other side of the river and the border, preventing any escape.

Audio message

"Clashes are continuing as a group of ISIS terrorists who are confined into a tiny area still fight back," Bali said.

With daily air strikes and shelling from the US-led coalition and its SDF ground partners, hundreds of fighters have been pummelled into submission in recent days.

The SDF said on Tuesday it had captured dozens of sick or wounded jihadists since gaining control of the camp.

The state, which IS supremo Abu Bakr al-Baghdadi proclaimed in mid-2014, once covered territory larger than Britain, straddling Syria and Iraq.

It administered millions of people, ran its own court system, coined its own currency, printed its own school books and instilled fear on a global scale.

It has crumbled under years of military operations in which Syrian and Iraqi forces backed by their international allies clawed back the land but left cities in ruins and populations homeless.

Baghdadi's whereabouts are unknown and the Baghouz holdouts are fighting over land the size of a few football pitches but IS spokesman Abi Hassan al-Muhajir came out of months of silence to spur on his troops.

"Avenge the blood of your brothers and sisters ... Set up the (explosive) devices, deploy the snipers," he said in an audio message released on IS's usual social media channels late on Monday.

Overcrowded camps

The loss of Baghouz, which looks a few days away at most, will mark the end of IS as land-controlling force but not as a potent military player in the region.

Many of its leaders and fighters in Iraq and Syria have long reverted to a guerrilla war, launching hit-and-run attacks from desert, mountain and other hideouts.

US President Donald Trump, who jumped the gun by more than a month on the end of the Baghouz battle, has ordered the complete withdrawal of US troops from Syria.

But on March 7, US Central Command head General Joseph Votel warned the fight against the extremists was "far from over" and stressed the need to maintain a "vigilant offensive" against the group.

The conflict in Syria entered its ninth year last week and the aid crisis spurred by the dying days of the IS 'caliphate' compounded an already bleak humanitarian picture.

The thousands of ghost-like people who streamed out of the last IS stronghold in recent weeks now fill overcrowded camps and prisons run by the Kurds further north.

The 70,000 people crammed into the biggest camp — Al-Hol — include more than 40,000 children, of dozens of different nationalities.

Some are unaccompanied, others are unwanted by their countries or origin, and many have witnessed beheadings or other forms of traumatising violence that will require urgent and specialised care.