The United Nations condemned on Tuesday the "weaponisation of food" in Gaza as a war crime and urged Israel's military to "stop shooting at people trying to get food".

"Israel's militarised humanitarian assistance mechanism is in contradiction with international standards on aid distribution," the UN human rights office said in written notes provided before a briefing.

"The weaponisation of food for civilians, in addition to restricting or preventing their access to life-sustaining services, constitutes a war crime."

That came as the head of the UN's Palestinian aid agency said that if funding is not found soon, he may have to take unprecedented decisions about the services it provides to Palestinian refugees.

"Cash flow is managed on a weekly basis," UNRWA chief Philippe Lazzarini said in Berlin.

"Without additional funding I will soon have to take unprecedented decision affecting our services to Palestinian refugees."

Lazzarini also said the new aid delivery mechanism in Gaza is an "abomination" and "a death trap".

Gaza's civil defence agency said Israeli forces killed 21 people waiting for aid near a distribution site in the centre of the Palestinian territory on Tuesday, the latest deadly incident targeting aid-seekers.

Civil defence spokesman Mahmud Bassal said 21 people were killed and around 150 wounded "as a result of the Israeli occupation forces' targeting of gatherings of citizens waiting for aid... in the central Gaza Strip with bullets and tank shells" in the early hours of Tuesday.

Israeli restrictions on media in the Gaza Strip and difficulties in accessing some areas mean AFP is unable to independently verify the tolls and details provided by rescuers and authorities in the Palestinian territory. (AFP/Reuters)