The US-backed Syrian Democratic Forces said it had begun the "final battle" to oust the Islamic State group from the last scrap of territory it holds in eastern Syria.
IS overran large parts of the country and neighbouring Iraq in 2014, declaring a "caliphate" and imposing a severe version of Islamic law, but after various military offensives, only a fragment remains.
Backed by air strikes by the US-led coalition against IS, the Kurdish-Arab alliance has in recent months cornered the jihadists in a final pocket of territory in Syria's eastern province of Deir Ezzor.
After a pause of more than a week to allow civilians to flee, the SDF said Saturday it had resumed the fight to seize the last four-square-kilometre (one-square-mile) patch from the jihadists.
"The SDF have launched the final battle to crush IS... in the village of Baghouz," the SDF said in a statement.
A gun-mounted IS vehicle fires at members of the US-backed Syrian Democratic Forces (SDF). Source: AAP
"After ten days of evacuating more than 20,000 civilians... the battle was launched tonight" to wipe out the last remnants of the organisation, it said.
SDF spokesman Mustafa Bali told AFP: "The battle has started."
"This battle will be sealed in the coming days," he added.
Bali said there could be up to 600 IS fighters still inside the pocket, most of them foreigners.
Hundreds of civilians are also believed to be inside.
"We have special units whose job it is to direct civilians to corridors they can cross" to safety, he said.
Near the battlefield, an SDF spokesman at the Omar oil field turned military base, said: "Progress is slow."
He said that when the force detects movement from IS fighters, it bombs them, but added: "There have not been any major changes."
Caliphate decimated
At the height of its rule, IS controlled territory the size of Britain.
But a series of separate military operations, including by the SDF, have left its proto-state in tatters.
On Thursday, the coalition said the "caliphate" had massively shrunk.
Major General Christopher Ghika, the coalition's deputy commander, described the size of the last IS pocket as "now less than one percent of the original caliphate".
More than 37,000 people, mostly wives and children of jihadist fighters, have fled IS territory since the SDF intensified its offensive in December, according to the Syrian Observatory for Human Rights.
The Britain-based war monitor has said that figure includes some 3,200 suspected jihadists.
The SDF launched an operation to expel IS from Deir Ezzor in September, and has slowly advanced against IS despite the jihadists putting up a fierce fightback.
In that time, more than 1,200 IS militants, more than 670 SDF fighters, and around 400 civilians have been killed in the fighting, the Observatory says.
Despite its caliphate being on the brink of collapse, IS still retains a presence in Syria's vast Badia desert and has carried out deadly attacks in SDF-held territory.
Syria's war has killed more than 360,000 people and displaced millions since it erupted in 2011 with the brutal repression of anti-government protests.
The country's Kurds have largely stayed out of the civil war, instead developing semi-autonomous institutions in northern and northeastern Syria.
The Kurdish People's Protection Units (YPG) have formed the backbone of the US-backed SDF, but neighbouring Turkey sees them as "terrorists".
The December announcement of a US troop pullout from Syria has seen Kurdish authorities scrambling to mend fences with the regime in Damascus.
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