Multiple aid workers were killed after a bus was attacked in Gaza on Wednesday night, according to a US-backed humanitarian aid organization which accused Hamas of carrying out the assault. Hamas has yet to respond to the allegations.

At least eight Palestinians who worked for the U.S-backed Gaza Humanitarian Foundation died in an ambush, the GHF said on Thursday, blaming Hamas militants for the killings that rocked the troubled food distribution operation.

The group, which has been harshly criticized by the U.N. and other aid providers, said at least five people had died in an attack on one of its buses.