US-backed fighters in Syria say they are meeting fierce resistance in the last enclave held by Islamic State (IS) militants near the Iraqi border.
A spokesman for the Syrian Democratic Forces (SDF) told the AP news agency "the most experienced" jihadists are defending their last stronghold.
Two years ago IS controlled large areas of Syria and Iraq.
But they are now holed up in a tiny pocket in Syria's eastern province of Deir al-Zour, near the Iraqi border.
On Saturday, after a pause of more than a week to allow some 20,000 civilians to leave the area, SDF spokesman Mustafa Bali said the group was launching the "final battle to crush IS" in the border village of Baghuz.
Overnight he told the Associated Press (AP): "The battle is very fierce. Those remaining inside are the most experienced who are defending their last stronghold. According to this you can imagine the ferocity and size of the fighting."
The SDF, backed by air strikes carried out of the US-led coalition, has driven out IS from towns and villages in north-eastern Syria in recent months.
At its peak in 2014, IS established a "caliphate" stretching across Syria and Iraq that was similar in size to the UK and ruled over more than 7.7 million people.
In December, US President Donald Trump said IS militants were "mostly gone" and announced the US would withdraw all of its 2,000 troops from Syria.
On Wednesday he said: "It should be announced, probably some time next week, that we will have 100% of the caliphate."
How many militants are left?
IS has suffered substantial losses, but the UN says it still reportedly controls between 14,000 and 18,000 militants in Iraq and Syria, including up to 3,000 foreigners.
Meanwhile, there are significant numbers of IS-affiliated militants in Afghanistan, Egypt, Libya, South-East Asia and West Africa, and to a lesser extent in Somalia, Yemen, Sinai and the Sahel.
Individuals inspired by the group's ideology also continue to carry out attacks elsewhere.