The diplomats, representing several Western diplomatic missions in Turkey, told the news outlet that some of the disenchanted Daesh members were fighters, while others were people who had been enticed to move to so-called Daesh caliphate.
Recently, Daesh has been losing ground and facing assaults on its stronghold of Raqqa in Syria.
The US authorities estimated earlier in the year that there were about 25,000 Daesh fighters in Syria and Iraq, which is 10,000 fewer than in 2015.
According to the New America foundation report, at the peak, there were around 4,500 fighters from the West among Daesh members.
Daesh is outlawed in many countries around the world, including the United States and Russia.