Saudi Foreign Minister Prince Faisal bin Farhan said it was "disheartening and absolutely unacceptable" that some wealthy countries had not brought their nationals home. 

"To those countries, you must step up, you must take your responsibility," he told a meeting of the international anti-ISIS coalition that was attended by US Secretary of State Antony Blinken.

The "caliphate", which ISIS proclaimed across swathes of Iraq and Syria in 2014, was declared defeated in 2019 following counter-offensives in both Iraq and Syria.

Thousands of militants and their family members continue to be held in detention canters and informal camps where US commanders have warned they could fuel an IS revival. 

Despite repeated calls for their repatriation, foreign governments have allowed only a trickle to return home, fearing security threats and domestic political backlash.

Blinken applauded countries, including Canada, that have brought home their nationals from Syria, urging other nations to follow suit. 

"Repatriation is critical" to reduce populations of large informal camps such as Syria's Al-Hol, which houses 10,000 foreigners, including ISIS relatives, he said.

"Failure to repatriate foreign fighters risks that they may again take up arms," he told coalition partners, pledging $148.7 million for stabilization efforts for Iraq and Syria.

The anti-ISIS coalition was formed in 2014 following the militants' lightning advance that saw reports of atrocities multiply as they overran non-Muslim as well as Muslim areas.

Despite its territorial defeat, ISIS militants continue to conduct attacks against civilians and security forces in both Iraq and Syria. 

The United Nations estimates that ISIS still has 5,000 to 7,000 loyalists across the two countries, roughly half of whom are fighters.

In April, the US-led coalition reported a significant drop in ISIS attacks in both Iraq and Syria since the start of the year.

ISIS attacks in Iraq dropped by 68 percent through April, while Syria saw a 55 percent decrease during the same period, the coalition said.