If they identified Hamas in the ambulance it's not a war crime. People just keep saying "war crime" over and over and clearly haven't actually read up on it.
I posted the articles earlier in the thread. You can not harbor terrorist, military, amo, weapons etc in humanitarian establishments to try to get immunity. All the opposing party has to do it give warning which Israel, at least on the surface, has given the impression they are doing with warnings, leaflets, and ordered evacuations.
It's terrible these people are dying but the hospitals in the north are all death traps at this point.