A spokesman for a U.S-backed force fighting the Islamic State group in Syria says intermittent clashes are taking place with the militants remaining in the last speck of IS-held territory in the country's east.

Some 300 IS militants, along with hundreds of civilians believed to be mostly their families, have been under siege for more than a week in the tent camp in the village of Baghouz.

Commanders of the Kurdish-led Syrian Democratic Forces say the militants are preventing the civilians from leaving.

Spokesman Mustafa Bali says sporadic SDF fire is meant to pressure the militants into allowing the last civilians to leave.

Recapturing Baghouz would mark an end to the territorial rule of the militants' self-declared "caliphate" that once stretched across a third of both Syria and Iraq.