The head of a controversial US-backed group preparing to move aid into the Gaza Strip has announced his abrupt resignation, adding fresh uncertainty over the effort's future.

In a statement by the Gaza Humanitarian Foundation (GHF), executive director Jake Wood said on Sunday he felt compelled to leave after determining the organisation could not fulfil its mission in a way that adhered to "humanitarian principles".

The foundation, which has been based in Geneva since February, has vowed to distribute some 300 million meals in its first 90 days of operation.

But the United Nations and traditional aid agencies have already said they will not cooperate with the group, amid accusations it is working with Israel.

The foundation has emerged as international pressure mounts on Israel over the conditions in Gaza, where it has pursued a military onslaught in response to the October 7, 2023, attacks by Hamas.

A more than two-month total blockade on the territory only began to ease in recent days, as agencies warned of growing starvation risks.

"Two months ago, I was approached about leading GHF's efforts because of my experience in humanitarian operations" Wood said.

"Like many others around the world, I was horrified and heartbroken at the hunger crisis in Gaza and, as a humanitarian leader, I was compelled to do whatever I could to help alleviate the suffering."

Wood stressed that he was "proud of the work I oversaw, including developing a pragmatic plan that could feed hungry people, address security concerns about diversion, and complement the work of longstanding NGOs in Gaza."

But, he said, it had become "clear that it is not possible to implement this plan while also strictly adhering to the humanitarian principles of humanity, neutrality, impartiality, and independence, which I will not abandon."

Wood called on Israel "to significantly expand the provision of aid into Gaza through all mechanisms" while also urging "all stakeholders to continue to explore innovative new methods for the delivery of aid, without delay, diversion, or discrimination".

His resignation came as an Israeli strike on a school housing displaced people in Gaza killed at least 20 people and injured dozens, local authorities said early on Monday.

Israel stepped up its military operations in the enclave early this month, saying it is seeking to eliminate Hamas' military and governing capabilities and bring back the remaining hostages who were seized in October 2023.

Medics said the dozens of casualties in the strike on the school, in the Daraj neighbourhood of Gaza City, included women and children. (AFP)