Seven staff from food aid charity World Central Kitchen, a group founded and run by Spanish-American celebrity chef Jose Andres, were killed in the besieged Palestinian territory on Monday.

Israeli Prime Minister Benjamin Netanyahu admitted Tuesday that his country's military has "unintentionally" targeted the group's convoy. Among the dead were Australian, British, Palestinian, Polish and US-Canadian employees.

The incident brings "the number of aid workers killed in this conflict to 196 -- including more than 175 members of our own UN staff," Guterres said in a speech to the UN General Assembly.

"This is unconscionable -- but it is an inevitable result of the way the war is being conducted," he said.

"It demonstrates yet again the urgent need for an immediate humanitarian ceasefire, the unconditional release of all hostages, and the expansion of humanitarian aid into Gaza -- as the Security Council demanded in its resolution."

Last week, the UN Security Council passed a resolution calling for a ceasefire -- thanks to an abstention from the United States, Israel's closest ally. The measure sparked ire from Netanyahu's government.

"The resolution must be implemented without delay," Guterres said.

Earlier, Guterres' spokesman Stephane Dujarric said the deaths had highlighted a "disregard for international humanitarian law and a disregard for the protection of humanitarian workers."