The UN has called on Israel to lift its blockade of humanitarian aid delivery into Gaza, saying blocking the desperately needed aid inflicts a "cruel collective punishment".

"International law is unequivocal: As the occupying power, Israel must allow humanitarian support in. Aid, and the civilian lives it saves, should never be a bargaining chip," Tom Fletcher, the UN relief chief, said in a statement on Thursday.

He said blocking aid starves civilians and leaves them without basic medical support, adding that it strips them of dignity and hope and "inflicts a cruel collective punishment. Blocking aid kills."

"The humanitarian movement is independent, impartial and neutral. We believe that all civilians are equally worthy of protection," said Fletcher, underlining that they remain ready to save as many lives as they can, despite the risks.

"But as the UN Secretary-General (Antonio Guterres) has made clear, the latest modality proposed by Israeli authorities does not meet the minimum bar for principled humanitarian support," said the statement.

Fletcher urged Israel to lift "this brutal blockade" and "let humanitarians save lives".

"To the civilians left unprotected, no apology can suffice. But I am truly sorry that we are unable to move the international community to prevent this injustice."

Since March 2, Israel has closed Gaza's crossings, blocking essential supplies from entering the enclave despite multiple reports of famine in the war-devastated territory.

The Israeli army renewed its assault on Gaza on March 18, shattering a January 19 ceasefire and prisoner exchange agreement with the Palestinian resistance group Hamas.

At least 52,400 Palestinians have been killed in Gaza in a brutal Israeli onslaught since October 2023, most of them women and children.