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

Witkoff comes with "a strong hope that we will come forward with another ceasefire as well as a humanitarian corridor for aid to flow, that both sides have in fact agreed to," State Department spokeswoman Tammy Bruce told reporters.

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

Israel says humanitarian aid is being allowed into Gaza and accuses Hamas of exploiting civilian suffering, including by stealing food handouts to sell at inflated prices or shooting at those awaiting aid.

'Hope and heartbreak'

In their statement, the humanitarian organizations 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 on Tuesday that 21 children had died due to malnutrition and starvation in the Palestinian territory in the past three days.

Standstill

Israel and Hamas have been engaging in drawn-out negotiations in Doha since July 6 as mediators scramble to end nearly two years of war.

But after more than two weeks of back and forth, efforts by mediators Qatar, Egypt and the United States are at a standstill.

More than two dozen Western countries recently urged an immediate end to the war, saying suffering in Gaza had "reached new depths".

Israel's military campaign in Gaza has killed 59,106 Palestinians, mostly civilians, according to the health ministry in the Hamas-run territory.

Hamas's Oct. 7, 2023 attack on Israel, which sparked the war, resulted in the deaths of 1,219 people, most of them civilians, according to an AFP tally based on official figures.