The United Nations says replacing its Palestinian relief agency UNRWA in Gaza and the West Bank is not the world body's responsibility, signaling it was Israel's problem, according to a letter excerpt seen by Reuters.

The U.N. formally responded in a letter to Israel's decision to cut ties with the United Nations Relief and Works Agency, a move that UNRWA has said leaves its operations in Gaza and the West Bank at risk of collapse.

Under a new law, Israel told the U.N. on Sunday it was ending a 1967 cooperation agreement with UNRWA that covered its protection, movement and diplomatic immunity. The law will also ban UNRWA's operations in Israel from late January.

"I would note, as a general point, that it is not our responsibility to replace UNRWA, nor do we have the capacity to do so," U.N. Secretary-General Antonio Guterres' chef de cabinet, Courtenay Rattray, wrote to a senior Israeli foreign affairs official late on Tuesday.

The mention of responsibility is a veiled reference to Israel's obligations as an occupying power.

The U.N. views Gaza and the West Bank as Israeli-occupied territory. International humanitarian law requires an occupying power to agree to relief programs for people in need and to facilitate them "by all the means at its disposal" and ensure food, medical care, hygiene and public-health standards.

Israel's mission to the United Nations did not immediately respond to a request for comment on Rattray's letter.

"If UNRWA is no longer able to operate it would be the responsibility of the Israeli authorities to replace its services that it delivers to civilians, in education, in health, and all sorts of other areas," U.N. spokesperson Stephane Dujarric later clarified to reporters.

Top U.N. officials and the Security Council describe UNRWA as the backbone of the aid response in Gaza, where Israel and Palestinian militants Hamas have been at war for the past year, leaving the enclave in ruins and on the brink of famine.

"UNRWA may be defined by a single word – failure," Israel's U.N. Ambassador Danny Danon told a meeting of the General Assembly on UNRWA on Wednesday. "This idea that UNRWA could not be supplemented is absurd."