"Islamic State of Iraq and Syria" (ISIS) elements defending their last dreg of territory in Syria will be "killed in battle" if they do not surrender, the Syrian Democratic Forces (SDF) said Tuesday (February 19th) ahead of a final showdown.

The Arab-Kurd alliance said they are trying to evacuate civilians trapped in the last half a square kilometre of ISIS-held territory before storming the extremists' holdout, AFP reported.

"We are working on secluding and evacuating civilians and then we will attack. This could happen soon," spokesman Mustafa Bali said, declining to provide more details on the operation.

ISIS fighters "have only two options, either they surrender or they will be killed in battle", he said.

An informed source told AFP that ISIS fighters are seeking safe passage to the city of Idlib in north-western Syria.

"They want to take remaining civilians with them as human shields. But the SDF are not willing to discuss this," said the source, asking not to be named.

The Syrian Observatory for Human Rights said the SDF have turned down the request.

AFP could not confirm this with an SDF official, but a commander with the alliance said ISIS has no leverage to negotiate.

"They are besieged in a very tight area and they have no other choice but to surrender," said the commander.

A small hamlet of buildings in the village of al-Baghouz is all that is left of the group's proto-state, which at its height spanned an area the size of the UK.

The UN on Tuesday expressed concern over "the situation of some 200 families, including many women and children, who are reportedly trapped" in the ISIS holdout.

"Many of them are apparently being actively prevented from leaving by ISIS," the UN said in a statement.