Jihadist fighters defending the last dreg of the Islamic State group's "caliphate" Saturday were holed up in half a square kilometre in a village of eastern Syria.
US President Donald Trump said the fall of the IS proto-state would be announced Saturday, but a top Syrian commander said his forces had slowed down their advance to protect civilians.
The jihadists declared a "caliphate" in large parts of Syria and neighbouring Iraq in 2014, but have since lost all of it but the tiny patch in eastern Syria near the border with Iraq.
"IS is besieged in a neighbourhood that is estimated to be 700 metres (yards) long and 700 metres wide" in the village of Baghouz, said Syrian Democratic Forces (SDF) commander Jia Furat.
"Baghouz is within our firing range but we are moving cautiously considering there are civilians still trapped there as human shields," said Furat, the overall commander for the operation.
"In a very short time, not longer than a few days, we will officially announce the end of IS's existence," he told reporters at the Al-Omar oil field turned SDF base.
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Thousands of people have flooded out of Baghouz over the past week -- mostly women and children related to IS fighters, but also suspected jihadists.
But an SDF spokesman said there were "still civilians inside in large numbers".
"We weren't expecting this number, otherwise we wouldn't have resumed the campaign four days ago. This is why it's been delayed," SDF spokesman Adnan Afrin told AFP.
Trump at around 1600 GMT on Friday said announcements over "the eradication of the caliphate" would be made "over the next 24 hours".
On Saturday, US-led coalition spokesman Colonel Sean Ryan acknowledged that the timeline had slipped because of the presence of civilians inside.
"There has been lapses as we continue to see hundreds of civilians still attempting to flee to safety," he said.
"The area of Baghouz has many tunnels, which slows operations," he added.
He said fighters of the Kurdish-led SDF were working to remove improvised explosive devices from the area, while keeping a lookout for possible IS suicide bombers.
Human Rights Watch called on commanders not to try to accelerate the offensive to suit Trump's timetable.
"The tempo of battle must not be dictated by political imperatives -- it must first of all protect civilians and possible hostages," HRW's director of counterterrorism, Nadim Houry, told AFP.
The Syrian Observatory for Human Rights said IS fighters were surrendering in large numbers to the advancing SDF fighters.
The SDF have been closing in on holdout jihadists since September.
The speck of terrain in Baghouz is all that's left of their self-proclaimed "caliphate" that in 2014 spanned an area the size of the United Kingdom and administered millions of people.
Successive offensives in Iraq and Syria have shattered the proto-state, which has since late 2017 been confined to the Euphrates Valley.
Trump's promise of a victory declaration came after he shocked allies and senior figures in his own administration with a December announcement that he had ordered a full troop withdrawal from Syria because IS had been "beaten".
That plan, which prompted the resignation of then defence secretary Jim Mattis, is set to be accelerated following a declaration of victory.
Beyond Baghouz, IS still has thousands of fighters and sleeper cells scattered across several countries.
In Syria, it retains a presence in the vast Badia desert, and has recently claimed deadly attacks in SDF-held territory.
The US Department of Defence has warned that without sustained counterterrorism pressure, IS could resurge within months.
US Vice President Mike Pence vowed Saturday at a security conference in the German city of Munich that the United States would "hunt down" IS remnants even after a military pullout.
"The United States will continue to work with all our allies to hunt down the remnants of ISIS wherever and whenever they rear their ugly heads," Pence said, using an alternative acronym for IS, but providing no further details.
A US withdrawal risks leaving Syria's Kurds exposed to a long threatened attack by neighbouring Turkey, who views Kurdish fighters as "terrorists".
To prevent this, they have scrambled to seek a new ally in the Damascus government after spending most of Syria's civil war working towards self-rule.
Eight years into a war that has killed more than 360,000 people, President Bashar al-Assad's government controls nearly two-thirds of the country.
German Chancellor Angela Merkel warned in Munich that a US pullout risks allowing regime allies Russia and Iran to boost their role in Syria.