Fires raged to the sound of gunfire and exploding artillery shells overnight in the Islamic State enclave of Baghuz in what U.S.-backed forces hope will be the last night for the terror group’s self-proclaimed caliphate.

Syrian Democratic Forces have been closing in on the last shred of IS territory in northeastern Syria, advancing slowly due to the presence of mines and booby traps, encountering what they describe as fierce resistance.

But by midday Tuesday, SDF officials were optimistic the caliphate’s death knell was near.

“The operation is over, or as good as over, but requires a little more time to be completed practically,” SDF spokesman Kino Gabriel told al-Hadath TV.

Earlier in the day, SDF spokesman Mustafa Bali said the U.S.-backed forces had advanced against IS in Baghuz on multiple axis, killing 38 IS fighters, even as they fought back, launching two rockets of their own.

Bali said IS positions in Baghuz were also pounded by 20 coalition airstrikes, which destroyed numerous defensive fortifications, weapons depots and several command posts.

Despite the heavy fighting, he said the SDF had suffered minimal casualties, with only three killed and another 10 wounded.

The SDF launched its final assault on the IS enclave late Sunday after pausing operations for nearly a week to allow more than 20,000 civilians, many related to IS fighters, to evacuate Baghuz for displaced persons camps. Hundreds of fighters also surrendered.

Smoke rises as U.S.-backed Syrian Democratic Forces (SDF) battle Islamic Stat
Smoke rises as U.S.-backed Syrian Democratic Forces (SDF) battle Islamic State militants remaining in a pocket of Baghuz, eastern Syria, March 11, 2019.

Officials with both the SDF and the United States were expecting a difficult battle over the collection of tents and wrecked buildings spanning no more than 1.6 square kilometers.

IS has been using a network of tunnels, caves and trenches to hide and launch counter attacks. U.S. and SDF officials described the system as complex, extending possibly for more than two kilometers, and rigged with explosives and booby traps.

Sources close to SDF leadership told VOA late Monday that although the battle for Baghuz was entering its final stages, there was concern the tunnel network might be even more extensive than initially thought.

They had also voiced concern about how many IS fighters were left to defend the last shred of the terror group’s self-declared caliphate, saying despite all the evacuations, hundreds of fighters, or more, might still be preparing to make a final stand.

A senior U.S. defense official said many of those still in Baghuz had been moving with IS for years, sticking with the terror group battle after battle.

“They want to continue the fight even if there’s not a physical territory to protect,” the official said.

Other U.S. officials have warned that IS used the evacuation of thousands of civilians in order to buy time.

Fighters with the U.S.-backed Syrian Democratic Forces (SDF) check a makeshif
Fighters with the U.S.-backed Syrian Democratic Forces (SDF) check a makeshift camp for Islamic State (IS) group members and their families in the town of Baghuz, eastern Syria, March 9, 2019.

The commander of U.S. forces in the Middle East, Gen. Joseph Votel, told U.S. lawmakers last week the evacuations were not a surrender but a “calculated decision” and part of a strategy to allow the terror group to complete its transition to a clandestine insurgency.

In an audio recording posted online Sunday, an IS fighter claiming to be in Baghuz called on other followers to find inspiration in the fight and take action.

“My brothers in Europe and in the whole world … be fierce and hard on the Crusaders,” the fighter said, according to a translation by the SITE Intelligence Group. “Kill them and displace them in revenge for your religion and your dignity.”

In a video, likely recorded in February in Baghuz, another fighter says death would be a “victory” for the last of the IS defenders as they never lost their faith.

Despite losing administrative control over almost all the land it once held in Syria and Iraq, U.S. defense officials warn IS still has “tens of thousands” of fighters working either as part of sleeper cells or as part of an active, clandestine insurgency.

A series of reports issued starting last year warned IS could have as many as 30,000 followers and fighters in Syria and Iraq, with officials cautioning they remain “well-positioned” to rebuild a physical caliphate.