And the World Health Organization (WHO) said it had received reports of further 20 also killed while waiting for food supplies.
Earlier on Tuesday, medics said residents had seen Israeli tanks firing shells at crowds of desperate Palestinians awaiting aid trucks along the main eastern road in Khan Younis.Â
They said at least 51 people were killed and 200 wounded, with at least 20 of them in critical condition.
Witnesses said Israeli tanks fired at least two shells at thousands of people awaiting aid trucks. Nasser Hospital wards were crowded with casualties, and medical staff had to place some on the ground and in corridors due to the lack of space.
Later, a WHO trauma surgeon and emergency officer Thanos Gargavanis said there were reports of a second mass casualty: "This is again the result of another food distribution initiative."
Israel's military has made no immediate comment on the incident, the latest in nearly daily mass deaths of Palestinians who were seeking aid in past weeks, including near sites operated by the US-backed Gaza Humanitarian Foundation.
"There's a constant correlation with the positions of the four announced food distribution sites and the mass casualty incidents," Gargavanis said, saying the trauma injuries in recent days were mostly from gunshot wounds.
Local health officials said at least 23 people were killed by Israeli gunfire on Monday as they approached a GHF aid distribution site in Rafah in the southern Gaza Strip.
The GHF stated in a press release late on Monday that it had distributed more than three million meals at its four distribution sites without incident.
There was no immediate comment from the Israeli military about Monday's reports of shootings. In previous incidents, it has occasionally acknowledged troops opening fire near aid sites, while blaming militants for provoking the violence.
Israel has put responsibility for distributing much of the aid it allows into Gaza into the hands of the GHF, which operates sites in areas guarded by Israeli troops.
The United Nations has rejected the plan, saying GHF distribution is inadequate, dangerous and violates humanitarian impartiality principles.
The latest bloodshed in the decades-old Israeli-Palestinian conflict was triggered in October 2023, when Palestinian Hamas militants attacked Israel, killing 1200 and taking about 250 hostages, according to Israeli allies.
U.S. ally Israel's subsequent military assault on Gaza has killed nearly 55,000 Palestinians, according to Gaza's health ministry, while internally displacing nearly Gaza's entire population and causing a hunger crisis.
Half a million people in the Gaza Strip face starvation, a global hunger monitor said last month.
The escalation is taking place as Palestinians in the Gaza Strip watch the exchange of attacks between Israel and Iran, which began with Israel launching major strikes on Friday.
Residents of the Gaza Strip have circulated images of wrecked buildings and charred vehicles hit by Iranian missiles in Israeli cities, and some were hopeful the wider conflict could eventually bring peace to their ruined homeland.
"We live these scenes and pain daily. We are very happy that we saw the day when we saw rubble in Tel Aviv, and they are trying to get out from under the rubble and the houses that were destroyed on top of their residents," said Gaza man Saad Saad.
Others said Iran's response was greater than many, including Israel, had expected.
"We saw how Iran ... lost patience and the time has come for Iran to teach the Israeli occupation state a lesson," said another Gaza man, Taysseir Mohaissan.
With Israel saying its operation could last weeks, fears have grown of a regional war dragging in outside powers.
