"Coalition members must take back the thousands of foreign terrorist fighters in custody, and impose accountability for the atrocities they have perpetrated," Pompeo said in Washington.

He was speaking at the opening of a meeting of foreign ministers from the global coalition to defeat Islamic State.

IS has lost almost all of its territory in Iraq and Syria. Former leader Abu Bakr al-Baghdadi was killed in a US raid last month, but experts and allies worry that the militant group remains a security threat in Syria and beyond.

Some 10,000 Islamic State detainees and tens of thousands of family members - including Australians - remain in camps and prisons in northeast Syria guarded by the Syrian Kurdish allies of the US.

Washington is pushing countries to take their citizens back, but so far they have been reluctant to do so.

Pompeo also asked coalition members to help fill the gap in funding to restore essential services and rebuild critical infrastructure in Iraq to facilitate the return of millions of displaced Iraqis.

"We will need help likewise in northeast Syria," he said on Thursday.

Since his announcement last December that he would pull all US troops out of Syria, US President Donald Trump has softened plans after a domestic backlash.

Pompeo also said on Thursday there were growing concerns about the threat of Islamic State outside of Iraq and Syria, and in places like West Africa and the Sahel.

He said the US was planning to hold a specific coalition meeting to focus on boosting support for those areas.

Australian Associated Press