U.S.-backed Syrian Democratic Forces (SDF) fire on Islamic State militant positions in Baghouz, Syria. Picture: AP Photo/Maya Alleruzzo

Baghouz— Besieged by US-backed forces in their last foothold in eastern Syria, Islamic State militants are calling on supporters across the world to stage attacks in their defence, according to a newly released audio recording.

The recording, purportedly from Isis, came as the Kurdish-led Syrian Democratic Forces continued to face stiff resistance Tuesday from Isis gunmen as US-backed fighters try to push deeper into the village of Baghouz on the eastern bank of the Euphrates River.

The brief, minute-and-a half recording, released by Isis supporters on social media and reported by the SITE Intelligence Group late on Monday said that men, women and children in Baghouz are being subjected to a "holocaust by the Crusaders," which is militant jargon for the US-led coalition against Isis.

In the audio, an unidentified Isis militant calls on Muslim "brothers, in Europe and in the whole world" to "rise against the Crusaders and ... take revenge for your religion." As the man speaks, cracks of gunfire can be heard in the background, apparently meant to suggest that he is in Baghouz.

"Crusaders warplanes" and "Kurdish atheists" are attacking his people, the man says. The recording's authenticity couldn't be independently verified.

The U.S.-backed forces resumed their offensive against the Islamic State group on Sunday night, after thousands of civilians and hundreds of fighters left the last sliver held by the extremists. Since then, 38 militants and three SDF fighters have been killed, according to SDF spokesman Mustafa Bali.

Argish al-Deiri, an SDF commander in Baghouz who identified himself by his nom de guerre told The Associated Press on Tuesday that his men advanced overnight and secured some positions on the edge of a tent settlement where Isis militants are making their last stand.

"We entered the camp, then stopped," he said. "There was resistance, and we withdrew. The planes struck the ammunition depot," setting off explosions that halted the push.

He said Isis militants were fighting back with heavy weapons, rifles and sniper fire, forcing SDF fighters to slow their advance rather than advancing quickly so that "you don't lose your men."

He said he hoped the Isis fighters will surrender in the coming "few days."

The capture of Baghouz would be a milestone in the devastating four-year campaign to defeat Isis's so-called "caliphate," which once covered a vast territory straddling both Syria and Iraq.

The Associated Press