IS final enclave in eastern Syria was pounded with air strikes and artillery Monday in a U.S.-backed assault aimed at wiping out the last pocket of its territory that once covered a third of Syria and Iraq.

Fires raged after dark in the enclave at Baghouz near the Iraqi border as it was targeted with rockets and thick plumes of smoke rose from the area. Heavy gunfire could be heard and flares lit up the night sky.

The U.S.-backed Syrian Democratic Forces (SDF) said earlier its fighters were encountering sniper fire and landmines and advancing slowly to avoid losses. The most hardened IS foreign fighters are still holed up in the area, the SDF has said.

Air strikes destroyed IS stores and vehicles, the SDF said.

A U.S. defense official, speaking on the condition of anonymity, said the U.S. did not believe senior Islamic State leaders were still in Baghouz.

“We’re pretty confident the leadership is not still down in this tiny little, basically hellhole that remains there,” the official said.

“Most of those folks have moved elsewhere as part of our assessment of their movement toward a more clandestine insurgency or preparing for the next fight when they don’t control territory,” the official added.

SDF commander Adnan Afrin said many jihadist fighters had been killed or wounded since the assault began Sunday.

IS fighters attempted four suicide attacks but the SDF captured an arms dump, said Mustafa Bali, head of the SDF media office. One SDF fighter was killed and four wounded.

The SDF has held off from a full assault for most of the past few weeks as many thousands of people poured from the enclave, including surrendering fighters, IS supporters, other civilians and some of the group’s captives.

(SD-Agencies)