Palestinians say Israeli forces opened fire on people trying to access a U.S.-backed group's aid distribution hub in Gaza, in the deadliest of such incidents reported to date.

The UN human rights chief said Israel's "means and methods of warfare" were inflicting "horrifying, unconscionable suffering".

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.