Nearly 400,000 Gaza residents have been displaced in the weeks since Israel resumed the war on the Palestinian territory, with relentless attacks leading to "large-scale civilian casualties," the UN secretary-general's spokesman said Monday.

"Survivors across Gaza are being displaced repeatedly and forced into an ever shrinking space where their basic needs just cannot be met," said spokesman Stephane Dujarric.

"Overall, we estimate that nearly 400,000 people have been displaced yet again since the breakdown of the ceasefire."

Much of the Gaza Strip's population of roughly 2.4 million people had already been displaced at least once between October, 2023, when the war began, and the start of the short-lived ceasefire in January.

In a joint statement, the heads of the UN Office for the Coordination of Humanitarian Affairs, World Health Organization, UNICEF and World Food Programme appealed "to world leaders to act -- firmly, urgently and decisively -- to ensure the basic principles of international humanitarian law are upheld."

Vast numbers of Gazans are "trapped, bombed and starved again, while, at crossing points, food, medicine, fuel and shelter supplies are piling up, and vital equipment is stuck" amid an Israeli humanitarian and commercial blockade in its second month, they added.

"We are witnessing acts of war in Gaza that show an utter disregard for human life," the agency chiefs said.

"Assertions that there is now enough food to feed all Palestinians in Gaza are far from the reality on the ground, and commodities are running extremely low."

Israel has dramatically expanded its footprint in Gaza since relaunching its war on the Strip last month. It now controls more than 50 percent of the territory and is squeezing Palestinians into shrinking wedges of land.

It has killed also at least 1,391 Palestinians and wounded 3,434 other since it resumed its genocidal war on 18 March, bringing the overall death toll among since October 2023 to 50,752, mostly children and women.