Hundreds of people have trudged out of the Isis' last stronghold, surrendering to US-backed forces before their final assault to capture the only village still in the militants' hands.

Some of Isis' most die-hard fighters are pinned down in Baghouz, a remote hamlet nestled on a bend of the Euphrates River close to the Iraqi border.

There is only one path out of what they once called the caliphate, snaking through the green grass and flowers of eastern Syria. No longer spanning an area the size of Britain, their territory is now visible in its entirety from hills that surround it, covering no more than a square mile.

After three days of fighting earlier in the week, the combat quieted - with a lull in artillery fire and US airstrikes - as the US-supported Syrian Democratic Forces gave the village's remaining inhabitants a chance to flee or give themselves up.

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More than 1200 people had accepted the offer since yesterday, American aid workers said, walking in the darkness toward the SDF militia fighters on the other side of a hill - and on to an uncertain future. Among those departing the village were defeated foot soldiers of Isis (Islamic State).

"Last night looked like a major break," said Dave Eubank of the Free Burma Rangers, a Christian group that specialises in delivering aid in war zones. "To have more than a thousand walking over pretty much at once, someone had to open the door."

More than 38,000 people have left Isis' shrinking territory in eastern Syria since the start of the year, monitoring groups say, but several thousand remain inside Baghouz, packed into tunnels under the village. Some of the most battle-hardened are still believed to be inside, using civilians as human shields and determined to fight to the death.

At its height, the self-proclaimed caliphate had been home to about 40,000 foreign fighters from 110 countries, according to the Soufan Centre, a New York-based research group focusing on strategic threats.

Almost five years after the militant group's lightning sweep across parts of Iraq and Syria, Isis is surrounded on all sides and the SDF expects to declare victory within days. At a nearby base, the SDF has built a stage for the anticipated victory announcement.