The US has said it expects “all parties on the ground” in Gaza to comply with international humanitarian law but declined to confirm whether it was carrying out its own assessment into the killing by the Israeli military of 15 people – paramedics, civil defence workers and a UN official.

Asked about the killings, state department spokeswoman Tammy Bruce said: “Every single thing that happens in Gaza is happening because of Hamas.”

The UN’s humanitarian agency has said five ambulances, a fire truck and a UN vehicle were struck “one by one” on 23 March and that 15 bodies, including paramedics still in their uniforms, had been gathered and buried in a mass grave.

The Israeli military said its troops had fired on vehicles “advancing suspiciously” without headlights or emergency signals and that a Hamas operative and other militants were among those killed, but it did not offer any comment on the accounts of bodies being gathered up and buried in the sand.

International humanitarian law prohibits the targeting of civilians and calls for specific protections for medical personnel.

The US, Israel’s biggest arms supplier, is also bound by its own laws prohibiting its weapons being used by foreign militaries in breach of humanitarian law.

Jonathan Whittall, the head of the UN’s humanitarian agency in Gaza, said the mass grave had been “marked” with an emergency light from one of the ambulances hit in the strike.

“It’s an absolute horror what has happened here,” he said in a video on X, adding that “healthcare workers should never be a target”.

Israel renewed its air and ground campaign in Gaza on 18 March after negotiations over a ceasefire deal with Hamas stalled.

More than 1,000 people have since been killed in Gaza since then, according to the territory’s Hamas-run health ministry.

The Israeli military launched a campaign to destroy Hamas in response to an unprecedented cross-border attack on 7 October 2023, in which about 1,200 people were killed and 251 were taken hostage.

More than 50,350 people have been killed in Gaza during the ensuing war, according to the health ministry.