Capturing the village in eastern Syria will be a milestone in international efforts to roll back the jihadists, whose self-styled "caliphate" covered roughly one third of Syria and Iraq at its height in 2014.

But it is universally accepted that the group, which has been in territorial retreat since then and suffered its major defeats in 2017, will remain a security threat as an insurgent force with sleeper cells and some desolate pockets of territory.

The SDF had said it expected a "decisive battle" on Sunday after advancing gradually for 18 hours to avoid land mines sown by Islamic State (IS), whose fighters are also using underground tunnels to stage ambushes and then disappear.

By midday, however, there was no sign of the battle being over, and a spokesman for the U.S.-led international coalition supporting the SDF said the pace of the advance had ebbed.

"ISIS fighters have been using suicide vests and car bombs to slow down the SDF offensive and hide from Coalition strikes in the area of Baghuz," Col. Sean Ryan told Reuters.

"They still hold civilians and are lacing the tunnels with IEDs as well," he said, referring to improvised explosive devices.

The SDF has previously estimated several hundred IS insurgents to be inside Baghouz, mostly foreigners, and the coalition has described them as the “most hardened” militants. But Ryan said their hiding underground made it difficult to determine current numbers.

From a position about 3 km from the front line on Sunday afternoon, warplanes and the pounding of artillery could be heard overhead as plumes of smoke rose over Baghouz.

An SDF commander there told Reuters that IS had sent explosive-ridden vehicles towards advancing fighters the night before. Air strikes destroyed two of them, and the SDF fired on a third to blow it up, he said.

The jihadists also shelled the approaching force.

Sinjar Shammar, from the Kurdish YPG which spearheads the SDF coalition, was wounded when shrapnel from a shell struck the armoured vehicle he was driving.

"My comrade was sent to the hospital. His leg is gone," said Shammar, 22, as his arm was being bandaged at a first-aid point.

"[But] morale is great [at the front line]. I will return to the comrades in a bit... God willing, we will triumph."

After declaring a modern-day caliphate across large swathes of territory it had seized in flash offensives in Syria and neighboring Iraq, IS attracted thousands of foreigners to live under its rule and defend its realm.

But its rapid expansion also attracted an array of enemies, both local and international, who began rolling back those gains. As IS territory shriveled, thousands of fighters, followers and civilians retreated to Baghouz, a small cluster of hamlets and farmland along the Euphrates River.

Over the last few weeks, they poured out in greater numbers than expected, holding up the final assault. An SDF commander told Reuters on Thursday that many of the people leaving the enclave had been sheltering underground in caves and tunnels.

After extricating the remaining civilians, the SDF resumed its assault on Friday evening. It has not ruled out the possibility that some militants have crept out, hidden among non-combatants.

The SDF commander-in-chief said on Thursday that his force would declare victory within a week. He was later contradicted by US President Donald Trump, who said the SDF had retaken 100 percent of the territory once held by IS.

Washington has about 2,000 troops in Syria, mainly to back the SDF against IS. Trump announced in December he would withdraw all of them, but the White House partially reversed itself last month, saying some 400 troops would stay.