More than 100 aid organisations warned on Wednesday that "mass starvation" was spreading in Gaza ahead of the US top envoy's visit to Europe for talks on a possible ceasefire and an aid corridor.

Israel is facing mounting international pressure over the catastrophic humanitarian situation in the enclave, where more than two million people face severe shortages of food and other essentials after 21 months of conflict, triggered by Hamas' attack on Israel.

The UN said on Tuesday that Israeli forces had killed more than 1,000 Palestinians trying to get food aid since the US- and Israel-backed Gaza Humanitarian Foundation started operations in late May – effectively sidelining the existing UN-led system.

A statement with 111 signatories, including Doctors Without Borders, Save the Children and Oxfam, warned that "our colleagues and those we serve are wasting away".

The groups called for an immediate negotiated ceasefire, the opening of all land crossings and the free flow of aid through UN-led mechanisms.

It came a day after the United States said its envoy Steve Witkoff will head to Europe this week for talks on Gaza and may then visit the Middle East.

Even after Israel began easing a more than two-month aid blockade in late May, Gaza's population is still suffering extreme scarcities.

In their statement, the humanitarian organisations said that warehouses with tonnes of supplies were sitting untouched just outside the territory, and even inside, as they were blocked from accessing or delivering the goods.

"Palestinians are trapped in a cycle of hope and heartbreak, waiting for assistance and ceasefires, only to wake up to worsening conditions," the signatories said.

"It is not just physical torment, but psychological. Survival is dangled like a mirage," they added.

"The humanitarian system cannot run on false promises. Humanitarians cannot operate on shifting timelines or wait for political commitments that fail to deliver access."

UN Secretary-General Antonio Guterres said Tuesday that the "horror" facing Palestinians in Gaza under Israeli military attack was unprecedented in recent years.

The head of Gaza's largest hospital said Tuesday 21 children had died due to malnutrition and starvation in the past three days. (AFP)