The tiny hamlet of Baghouz in northern Syria was the scene of intense fighting for the third day on Monday, as US-backed Syrian forces advanced on the final ISIS-held pocket in the country.

During the day 13 ISIS fighters, including five suicide attackers, were killed as well as six fighters from the Syrian Democratic Forces (SDF), according to the Syrian Observatory for Human Rights, a UK-based war monitor.

The SDF launched what it said was a final push against ISIS on Saturday and the capture of Baghouz would mean the end of a four-year long military campaign to recapture territory held by the extremist group, which once stretched across more than a third of both Iraq and Syria.

All that is now left of the “caliphate” declared by ISIS in 2014 is a cluster of farmhouses and fields on a bend of the Euphrates River in northern Syria. But holed up in tunnels, trenches, and houses around Baghouz are an estimated 600 diehard ISIS fighters.

More than 79 states and international organisations joined the coalition against ISIS but Monday’s fighting was being carried out by Kurdish-led SDF fighters with support from coalition warplanes and artillery. About 2,000 US soldiers are in Syria to “advise and assist” the SDF.

The presence of fighters’ wives and children has slowed the offensive, according the Observatory. Advancing SDF fighters have also faced sniper fire, car bombs and landmines.

SDF spokesman Mustafa Bali said “dozens” of his fighters were held hostage by ISIS in Baghouz, with the risk of their execution complicating the advance.

Another 600 people fled the fighting to reach SDF lines on Sunday, according to the Observatory. Since December, tens of thousands of mostly women and children related to ISIS fighters have crossed into SDF-controlled territory.

Among those who fled on Sunday were seven Turks, three Ukrainians and two French women, according to the Observatory.

ISIS leader Abu Bakr Al Baghdadi is not believed to be in Baghouz, Mr Bali said, and likely not in Syria.

While the SDF has warned that it is unable to detain growing numbers of ISIS fighters and their families long term, watchdog organisation Human Rights Watch has warned that the rendition of prisoners to Iraq to face trial risks breaching international and domestic laws.

The group says it is aware of at least five foreign ISIS suspects who were transferred from Syria to Iraq by US forces.

“The US may be breaking domestic law with these transfers,” senior US national security counsel at Human Rights Watch Laura Pitter told The National. “The Foreign Affairs Reform and Restructuring Act of 1998 (Farra) provides that it ‘shall’ be the policy of the US not to transfer people to places where they face a substantial risk of torture, regardless of whether the person is physically present in the US.”

The US maintains that transferred detainees do not face a substantial risk of torture in Iraq, a claim that Human Rights Watch disputes.

The fighting is expected to be over within days, Mr Bali said. Surviving ISIS fighters have sought refuge in the vast Syrian desert or elsewhere returned to civilian populations, where it is feared they may remain as “sleeper cells” to carry out insurgent attacks.

With the end of combat operations in sight, the US is just weeks away from withdrawing its troops from Syria, the top US commander overseeing American forces in the Middle East said on Sunday.

US Army General Joseph Votel said he expected American troops to remain stationed in neigbhouring Iraq but did say: "We don't want to keep people on the ground that we don't need and [who] don't have a valid mission."

US President Donald Trump ordered the withdrawal of US troops from Syria in December.