More than 100 Palestinians are reported to have been killed while waiting for aid to arrive in northern Gaza.
The Hamas-run health ministry blamed Israeli forces, and Palestinian media quoted medical sources as saying Israeli troops fired on the crowd.
An Israeli military source said troops opened fire because they believed the soldiers were "at risk."
An Israel Defense Forces (IDF) statement also said dozens were killed by being run over and hit by trucks.
A journalist in Gaza City told the BBC that Israeli tanks fired on a crowd that had come to pick up supplies. The Israeli military says it has no knowledge of shelling in the area and that the incident is under investigation.
Videos posted on social media show the dead in Gaza City being loaded onto empty aid trucks and a donkey cart.
This came hours before Gaza's health ministry announced that more than 30,000 people, including 21,000 children and women, had been confirmed killed since the start of the war between Israel and Hamas.
About 7,000 others have been reported missing and 70,450 have been treated for injuries over the past four months, according to the ministry.
The UN is also warning of a famine in the north of the territory, where some 300,000 people live with little food or clean water.
The Israeli military launched a wide-ranging air and ground campaign to destroy Hamas - which is banned as a terrorist organization by Israel, the UK and others - after the gunmen killed around 1,200 people in southern Israel on October 7 and they took another 253 people hostage. .