ISIS is down to its last few hundred fighters and less than a square kilometre of land left in Syria, although it may have up to 20,000 armed fighters in Syria and Iraq, US envoy James Jeffrey said on Friday.
"We are just about finished with the campaign along the Euphrates to defeat the last territorial holdings of the 'caliphate'," said Mr Jeffrey, the US special envoy to the global coalition to defeat ISIS .
Mr Jeffrey said the US was helping the Syrian Democratic Forces to secure ISIS prisoners but was also launching a campaign to have countries take back foreign fighters and their families, for prosecution or re-education.
ISIS redrew the map of the Middle East in 2014 when it declared its ultra-radical "caliphate" and established a rule known for mass killings, sexual enslavement and punishments such as crucifixion.
The militants suffered their major military defeats in 2017, when they lost the cities of Mosul in Iraq and Raqqa in Syria.
They were then forced down the Euphrates River to their last bastion at Baghouz, a cluster of hamlets on the eastern bank.
"We believe that there's between 15,000 and 20,000 Daesh armed adherents active, although many are in sleeper cells, in Syria and in Iraq," Mr Jeffrey said.
Speaking on a video link after a Syria humanitarian conference in Brussels, Mr Jeffrey said the struggle to defeat ISIS ideology would continue and there was no timetable for a full US withdrawal from Syria.
Some troops would be pulled out but a contingent would stay in north-eastern Syria, backed by coalition partners, to continue the fight and prevent a power vacuum developing.
The US would also maintain a force at Al Tanf, close to the Iraqi and Jordanian borders, to support local forces against ISIS.
With a smaller force and much less combat after the territorial defeat of IS in Syria, US costs would be far less, Mr Jeffrey said.
In 2018, US military operations in Syria cost about $2 billion from a total defence budget of $700bn.