A woman was killed and dozens were detained in a Kurdish-run camp in north-east Syria over a row about secret Sharia "courts".
Security incidents have been on the rise in the crowded camp of Al Hol, where the murder took place.
The camp houses families of ISIS fighters among its 70,000 residents, with foreign women and their children separated from Syrian and Iraqi inhabitants.
"One woman was killed and six others were wounded in the sector reserved for foreign women," an official of the Kurdish administration in north-east Syria told AFP.
He said security forces detained 40 women from the foreigners' section at the camp.
The source said ISIS women who set up the clandestine courts to try their peers had stabbed to death a woman who the camp's security personnel had been trying to rescue.
But local Kurdish news agency Anha gave a different account.
It said an intervention by the security forces in the foreigners' section prompted a gunfight with the female militants, killing one of the foreigners.
There was no independent confirmation of either version.
Mustafa Bali, spokesman for the Kurdish-led Syrian Democratic Forces which led the offensive against ISIS, on Monday warned of worsening conditions in Al Hol.
The situation is "deteriorating sharply", Mr Bali wrote on Twitter.
"Daesh militants have stepped up their regrouping efforts through women in the camp recently. This is going to be very dangerous in future."
The UN Office for the Co-ordination of Humanitarian Affairs said: "Tensions remain high in the camp with security incidents reported on a weekly basis."
In March, the SDF announced the defeat of the "caliphate" that ISIS declared in 2014 over parts of Syria and Iraq, after the force expelled the last terrorists from the border village of Baghouz.
Tens of thousands of people, mostly women and children, were trucked to Kurdish-run camps in north-east Syria during the weeks-long campaign.
About 12,000 foreign ISIS fighters and family members, 4,000 women and 8,000 children, are now living in such camps, Kurdish authorities say.
Some have been repatriated from their home countries in Europe and Asia.
But some countries, such as the UK, say it is too dangerous to take back fighters that have fled the country to join ISIS.
On Sunday, British Home Secretary Priti Patel said she rejected ISIS runaway Shamima Begum’s request to return to the UK.
The teenager fled east London aged 15 to join the terrorist organisation in 2015.