NEAR BAGHOUZ, Syria (REUTERS) - The US-backed Syrian force closing in on the ISIS' last territorial bastion near the Iraqi border expects a "decisive battle" on Sunday (March 3) after advancing slowly, a spokesman said overnight.

Capturing Baghouz, an eastern Syrian village on the bank of the Euphrates River, would cap four years of international efforts to roll back the extremists, but the Islamic State in Iraq and Syria group remains a threat, using guerrilla tactics and holding some desolate land further west.

Thousands of fighters, followers and civilians had retreated to this tiny cluster of hamlets and farmland in Deir al-Zor province as ISIS territory shrunk and over the last few weeks, they have poured out, holding up the final assault.

The Syrian Democratic Forces clashed with ISIS for nearly 18 hours inside Baghouz after removing the remaining civilians and resuming their assault on Friday evening.

"Our forces are advancing at a slow pace to avoid any problems since Daesh (ISIS) mined the area very extensively. Thousands of mines are present along the roads in that small patch," said Mr Marvan Qamishlo, a military media official from the Syrian Democratic Forces.

"We expect a decisive battle in the morning," he told Reuters on a hill overlooking Baghouz.

The remaining militants are mostly foreigners who are using tunnels to hide and launch surprise attacks against fighters from the Syrian Democratic Forces, Mr Qamishlo added.

The Syrian Democratic Forces has previously estimated several hundred ISIS insurgents to be inside, and the US-led international coalition supporting the forces has described them as the "most hardened" militants.

The Syrian Democratic Forces commander-in-chief said last Thursday that his force would declare victory within a week.