The U.S.-backed Syrian force closing in on Islamic State’s last territorial bastion near the Iraqi border expects a “decisive battle” on Sunday 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 jihadists, but the 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 IS territory shrunk and over the last few weeks, they have poured out, holding up the final assault.

The Syrian Democratic Forces (SDF) clashed with Islamic State (IS) 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 (IS) mined the area very extensively. Thousands of mines are present along the roads in that small patch,” said Marvan Qamishlo, an SDF military media official.

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

The remaining jihadists are mostly foreigners who are using tunnels to hide and launch surprise attacks against SDF fighters, Qamishlo added.

The SDF has previously estimated several hundred IS insurgents to be inside, and the U.S.-led international coalition supporting the SDF has described them as the “most hardened” militants.

The SDF commander-in-chief said on Thursday that his force would declare victory within a week.