More than 100 aid organizations warned on Wednesday that "mass starvation" was spreading in Gaza ahead of the U.S. 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 Palestinian territory, where more than 2 million people face severe shortages of food and other essentials after 21 months of conflict, triggered by Hamas's attack on Israel.

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