Secretary of State Antony Blinken on Monday pressed US allies to bring back citizens arrested overseas for joining the Islamic State group, warning they could not be held indefinitely in Syria.
Blinken made the appeal in Rome at a meeting of an 83-member coalition on defeating the extremist network, where he and the Italian hosts called for greater attention to the jihadist threat in Africa.
About 10,000 suspected Islamic State fighters are being held in northern Syria by Western-allied Kurdish fighters, according to US estimates.
"This situation is simply untenable. It just can't persist indefinitely," Blinken said.
"The United States continues to urge countries -- including coalition partners -- to repatriate, rehabilitate and, where applicable, prosecute their citizens," he said.
France and Britain, two of the closest US allies, have been holdouts against such calls which were also made by former president Donald Trump's administration.
Both nations have traumatic experiences with attacks and see little incentive and plenty of political cost to bringing back radicalised citizens who are already jailed overseas.
Blinken praised Italy as one of the few Western European nations to repatriate its citizens and also hailed efforts by Central Asian nations such as Kazakhstan, which he said had brought back 600 fighters and their family members and put them in rehabilitation programmes.
According to a Human Rights Watch report in March, the Kurdish-led Syrian Democratic Forces are holding more than 63,000 women and children of suspected Islamic State fighters from more than 60 countries in two camps surrounded by barbed wire.
In a joint statement, the coalition voiced "grave concern" over the plight of prisoners in Syria and said it was important to find "a comprehensive and long-term solution ".
Blinken said the United States was offering another $436 million, mostly through international organisations, to care for Syrians displaced in the brutal decade-old war including by supporting Covid vaccination.
- Stepping up efforts in Africa -
The Islamic State group has lost almost all of its territory in Syria and Iraq, where it once ran a vast self-styled caliphate marked by an extraordinary campaign of brutality against religious minorities and women.
But the extremists are exerting growing force in Africa including in the Sahel, where France is winding down a military campaign, and in Mozambique.
There was little talk of new military actions but Blinken said the coalition would put a new emphasis on Africa in efforts to cutting off financing.
"We fear that ISIS can regain strength and that is why we must not lower our guard," Italian Foreign Minister Luigi Di Maio told reporters.
"We must step up the action undertaken by the coalition, not by shifting our focus but by increasing the regions in which we operate -- not just the Middle East but Africa," he said.
German Foreign Minister Heiko Maas said the coalition "can and must work more closely together with African countries, and I think the participants here are in agreement about that".
The Central African Republic and Mauritania joined the coalition with the Rome meeting while Italy invited other African nations including Ghana as observers.
Blinken also announced that the United States was designating Ousmane Illiassou Djibo, a Niger-born Islamic State leader based in Mali, as a "specially designated global terrorist", making any transactions with him a crime under US law.
The coalition's ministers were meeting for the first time in person since February 2019, with diplomacy hampered by the coronavirus pandemic.
The coalition was also rocked by Trump's abrupt decision to pull troops out of Syria, declaring that the United States had completed its mission.