The UN's counterterrorism chief said that his office was told that 700 people died recently in two camps in northeast Syria, where more than 70,000 mainly women and children connected to ISIS fighters were held in "very dire conditions."
Vladimir Voronkov told a news conference on Thursday that the people, including children, died of "lack of medicine, lack of food" at the Al Hol and Roj camps, which are overseen by Kurdish-led forces allied with the United States who led the fight against the extremist group.
He said the deaths in the camps created "feelings of anger".
Mr Voronkov did not clarify when the 700 reportedly died or what the source of the information was.
The Kurdish Red Crescent said in January that 511 people died in the largest camp, Al Hol, in 2019. Child mortality in the overcrowded camps was high. So far, there has been no known outbreak of coronavirus in the camps. A UN team visited the largest site this month.
Mr Voronkov urged the international community to tackle "the huge problem" of what to do with these people, saying keeping them in camps "is very dangerous". He said that "they could create very explosive materials that could be very helpful for terrorists to restart their activities" in Syria and Iraq.
ISIS, which once controlled large parts of Iraq and Syria, lost its last Syrian strongholds in early 2019. Despite the loss of its self-styled caliphate, UN experts said this year that the extremist group was mounting attacks in Syria and Iraq and was planning to break its fighters out of detention.
In addition to the Al Hol and Roj camps, Kurdish fighters were guarding thousands of ISIS fighters and boys in prisons.
After ISIS militants lost control of the oil-rich northeast, Turkey invaded areas along its borders last October and now controls slivers of land in the region. There are tensions between the Turkish-allied fighters and Kurdish groups, which Ankara considers terrorists. In addition, hundreds of US troops remain in northeast Syria.
The International Crisis Group reported on April 7 that there were 66,000 women and children in Al Hol and 4,000 in Roj, most of them relatives of ISIS extremists, "but some former affiliates of the group themselves".
The Brussels-based think tank said that the majority were either Syrian or Iraqi, with the numbers roughly split, and about 13,500 were from other countries.
The group said humanitarian workers described the detention sites "as ridden with tuberculosis and perilously overcrowded, with one speaking of 'dramatic mortality rates'".
Mr Voronkov said: "No country would like to have these people back, with this very negative and very dangerous terrorist background."
He said there were about 9,000 children, and the first priority should be to save those under 6 years old, "because in this period of time children are absolutely not in the position to be indoctrinated".
Mr Voronkov said the UN Office of Counter-Terrorism was pushing the issue very strongly with countries whose citizens were detained. Only a small number were repatriating their citizens, including Central Asian countries, the United States and Russia, he said.
Women were "a more difficult story", Mr Voronkov said.
There were "victims of terrorism" who did not understand what they were doing when they accompanied the men in their families to Syria and Iraq, he said, "but there are a lot of radicalised women among detained people in camps".
Mr Voronkov said he believed the way forward was to prosecute the women and then rehabilitate and reintegrate them into society, but he conceded that that was "a very challenging issue".
