At least 67 Palestinians have been killed and more than 150 injured while waiting for aid in northern Gaza, the territory's Hamas-run health ministry has said.
It was one of the highest reported death tolls among recent cases in which aid seekers have been killed, including 36 on Saturday, according to Gaza’s health ministry.
Another six people were killed near another aid site in the south, it added.
The Israel Defense Forces (IDF) said that it had “fired warning shots” to remove what it said was "an immediate threat".
The military said it disputed the number of reported casualties and it "certainly does not intentionally target humanitarian aid trucks".
It did not immediately comment on the incident in the south.
The UN World Food Programme (WFP) said that shortly after entering Gaza, a WFP convoy of 25 trucks carrying food aid encountered "massive crowds of hungry civilians" who then came under gunfire.
"WFP reiterates that any violence involving civilians seeking humanitarian aid is completely unacceptable," it said in a statement.
In total, health authorities said 88 people had been killed by Israeli gunfire and airstrikes across the enclave on Sunday.
It comes as Israel’s military dropped leaflets urging people to evacuate from neighbourhoods in central Gaza’s Deir al-Balah.
Dozens of families began leaving their homes, carrying some of their belongings. Hundreds of thousands of displaced Gazans have been sheltering in the Deir al-Balah area.
Israel's military said it had not entered the districts subject to the evacuation order during the current conflict and that it was continuing "to operate with great force to destroy the enemy's capabilities and terrorist infrastructure in the area".
Israeli sources have said the reason the army has so far stayed out is because they suspect Hamas might be holding hostages there. At least 20 of the remaining 50 hostages in captivity in Gaza are believed to still be alive.
Hostage families demanded an explanation from the army.
"Can anyone (promise) to us that this decision will not come at the cost of losing our loved ones?" the families said in a statement.
Much of Gaza has been reduced to a wasteland during more than 21 months of war and there are fears of accelerating starvation.
Palestinian health officials said hundreds of people could soon die as hospitals were inundated with patients suffering from dizziness and exhaustion due to the scarcity of food and a collapse in aid deliveries.
"We warn that hundreds of people whose bodies have wasted away are at risk of imminent death due to hunger," said the health ministry, which is controlled by Hamas.
The United Nations also said on Sunday that civilians were starving and needed an urgent influx of aid.
