They're reputable-seeming, but their various web pages are notably sparse, and this type of rumor-monging seems outside their stated area of knowledge.
Sounds like disinformation to me.
ISSA is a private "NGO" run by Gregory Copley, who seem to be a bit of a conspiracy theorist. Copley was a vociferous opponent of the Kosovo campaign and reported that NATO sustained the following losses that were covered up by DOD: 38 combat aircraft, six helicopters, seven UAV's, "many" cruise missiles shot down and, get this, 12 US Green Beret casualties and 8 British SAS commandos all KIA. He also claimed a US Air Force pilot was secretly held as a POW.
I would not rely on this source without corroberation.