Panos Moumtzis, UN humanitarian coordinator for the Syria crisis, urged governments on Thursday not to abandon the 2,500 children held in a "restricted" section of the sprawling al-Hol camp in northeastern Syria.
"There is a prime responsibility of states vis-a-vis their own nationals," Moumtzis said at a news conference in Geneva, calling on home nations to take responsibility for repatriating their citizens.
"Really, nobody should be rendered stateless," said Moumtzis. "Children should be treated first and foremost as victims" and "irrespective of family affiliation", he added.
A number of European countries have refused to take back their citizens who joined ISIL after the group declared its caliphate in Iraq's second-largest city, Mosul, in 2014.
At its height, the armed group ruled one-third of both Syria and Iraq, and more than 30,000 foreigners, including 6,000 Europeans, travelled to the Middle East to join it.
There are 10,000 non-Syrian and non-Iraqi nationals from 30-40 countries at the sprawling Kurdish-run camp in Hassakeh province, according to the Red Cross.
The camp, established in the 1990s to accommodate about 5,000 Iraqi refugees, held 10,000 people last December. It is now overcrowded with more than 75,000 people, according to Moumtzis. More than 90 percent are women and children.
The huge influx followed the assault by the Kurdish-led and US-backed Syrian Democratic Forces (SDF) against the last ISIL holdout in Baghouz on the Euphrates River. The enclave collapsed last month.
SDF said it has rounded up more than 5,000 ISIL fighters, of whom 1,000 were foreigners, but said it does not have the "authority or the capacity" to prosecute them or hold their families long-term.
The United States and the SDF have long urged European allies to take back ISIL fighters but negotiations have so far ended without agreement and only a handful of detainees have been repatriated.
Home countries of suspected ISIL members are reluctant to take them or their families back because of potential security risks and likely public backlash.
Britain revoked the citizenship of a teenager who left at 15 to join ISIL in Syria, while Austria and Switzerland have said they will not help bring adults who joined ISIL home.
On Thursday, Moumtzis called for a "concerted effort" from the international community.
"This is not about blaming or 'naming and shaming', but it's really about being practical and finding a way forward that would find a solution," he said.
Meanwhile, aid agencies and officials have warned of a humanitarian emergency at al-Hol camp.
According to the UN, 260 people have died while making the journey or upon arrival at the camp since December, including 211 children under the age of five.