ISIS has made a desperate call to its followers to launch attacks and avenge the siege of its last enclave as the battle nears its end, an audio message released on Tuesday said.
And a Syrian Democratic Forces official said the fight against ISIS in Baghouz, eastern Syria, was close to being over on Tuesday after a wave of overnight bombardments.
The besieged piece of land is the last territory held by the militants, who have been driven from about a third of Iraq and Syria over the past four years by its enemies, including a US-led international coalition.
"The operation is over, or as good as over, but requires a little more time to be completed practically on the ground," SDF spokesman Kino Gabriel told Al Hadath TV.
The 90-second recording posted on Tuesday was circulated on social media by ISIS sympathisers and its details published by US monitoring company Site Intelligence.
It claimed its remaining fighters and members, including women and children, were being subjected to a “holocaust”.
The US military and allied countries are carrying out air strikes against the group’s last fighters.
The SDF, a Kurdish-Arab coalition, has reduced ISIS's Baghouz holding to little more than a 500-metre square area.
In the audio, an unidentified ISIS militant calls on Muslim "brothers, in Europe and in the whole world" to "rise 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.
The recording's authenticity could not be independently verified.
The US-backed forces resumed their offensive against the 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, the forces' spokesman Mustafa Bali said.
On Sunday, Mr Bali said it was unclear how many people remained inside the land on the banks of the Euphrates.
"We expect there to be from 1,000 to 1,500 terrorists inside," he said.
"During the advance, if our forces notice the presence of civilians our special units will do the necessary to bring them away from the clashes or even evacuate them".
ISIS has slowed the advance with landmines and booby traps, and suicide and car bomb attacks on the SDF.
The SDF is hoping that the remaining ISIS fighters surrender inside Baghouz so that they do not have to clear the entire area.
But a small number of fighters have raised the ISIS flag in the centre of Baghouz and continue to resist the SDF and coalition air strikes.
The group's defences include extensive tunnels and ISIS's most hardened foreign fighters are holed up there, the SDF said.
US-led coalition jets mounted 20 air raids that destroyed ISIS military vehicles, defensive positions, two ammunition stores and a command post.
Washington does not believe any senior ISIS leaders are in Baghouz, believing they have gone elsewhere as part of the group's shift towards an insurgency, a US defence official said.
ISIS still operates in remote territory elsewhere and it is widely assessed that it will continue to represent a potent security threat.
It has continued to claim deadly attacks in SDF-held territory in recent months, and the US military has warned of the need to maintain a "vigilant offensive".
The US is expected to keep 200 "peace-keeping" troops in Syria after the end of the offensive, despite President Donald Trump's shock announcement in December that all 2,000 American soldiers would leave.
Most of the people moved from Baghouz have been taken to a camp for internally displaced people in Al Hol, north-eastern Syria, where the UN says conditions are "extremely dire".
About 65,000 people now live in the camp, which the UN says was built to house 20,000.
Pro-Syrian government forces hold the opposite bank of the Euphrates across from Baghouz and Iraqi militias are stationed at the border, cutting off any easy escape for the militants.
The support voiced by many of them for ISIS, particularly among foreigners, has posed a complex security, legal and moral challenge for the SDF and the foreign fighters' governments.
Those issues were underscored on Friday with the death of the newborn son of Shamima Begum, a British woman who left to join ISIS when she was a schoolgirl, aged 15.
