'Fewer than 1,000 IS fighters left in Iraq, Syria'
The figure is a third of the number suggested earlier this month.
"Due to the commitment of the coalition and the demonstrated competence of our partners in Iraq and Syria, there are estimated to be less than 1,000 ISIS terrorists in our combined joint area of operations, most of whom are being hunted down in the desert regions in eastern Syria and Western Iraq," the US-led coalition told Reuters, using an alternative acronym for the extremist group.
Iraq and Syria both declared victory over IS in the past few weeks, after a year that saw the two countries' armed forces, foreign allies and tribal units drive the extremists out of towns they had controlled for three years.
The United States has led an international coalition conducting airstrikes against IS since 2014 when the group swept across a third of Iraq.
US troops served as advisers on the ground with Iraqi government forces and with Kurdish and Arab groups in Syria.
Assad's main ally Russia also said on Wednesday the main battle with Islamic State in Syria was over. Foreign Minister Sergei Lavrov said the key task in Syria was now destroying another Islamist group, the Nusra Front.
Most of the fighters had been killed or captured over the past three years, the coalition said on Wednesday.
It would not respond to a question on whether some fighters could have escaped to other countries, saying it would not "engage in public speculation" but said it was working on preventing that.
"We can tell you that we are working with our partners to kill or capture all remaining ISIS terrorists, to destroy their network and prevent their resurgence, and also to prevent them from escaping to bordering countries," it said.
This week, the UAE banned Tunisian women from boarding passenger planes after reportedly receiving intelligence that extremists were returning from Iraq and Syria and plotting terror attacks.
More fighters from Tunisia are fighting abroad than from any other Arab state.
Meanwhile, Britain's new Defence Minister Gavin Williamson said British citizens fighting for Islamic State should be located, killed and banned from returning to the country.
"Quite simply, my view is a dead terrorist can't cause any harm to Britain," he told the Daily Mail earlier this month.
"We should do everything we can do to destroy and eliminate that threat," he added.
Agencies contributed to this report