The U.N. Secretary-General Antonio Guterres called on Israel to allow critical humanitarian supplies into the blockaded Gaza Strip, where over 2 million civilians are currently being bombarded with nowhere to flee, as Egypt rejected a corridor to be established for civilians.

"Crucial life-saving supplies, including fuel, food and water must be allowed into Gaza," Guterres said.

"We need rapid and any unimpeded humanitarian access now," he urged.

The U.N.'s human rights chief, Volker Turk, said Tuesday that Israel's "full siege" of the Gaza Strip violates international law.

"The imposition of sieges that endanger the lives of civilians by depriving them of goods essential for their survival is prohibited under international humanitarian law," Turk said in a statement.

Guterres said U.N. personnel are on the ground in Gaza "working around the clock to support" civilians amid the spiraling violence, and about 220,000 Palestinians have taken shelter at 92 UNRWA facilities in the coastal enclave. UNRWA is the U.N.'s Palestine refugee agency.

"U.N. premises, and all hospitals, schools and clinics must never be targeted," he said. "I deeply regret that some of my colleagues have already paid the ultimate price."

Eleven UNRWA staffers have been killed by Israeli airstrikes in Gaza since Saturday, including some who died in their homes with their families, the agency said. Nearly two dozen of the agency's facilities have been affected by airstrikes, including two schools. Four Red Crescent workers were also killed by Israel in airstrikes in the Gaza Strip on Wednesday.

Meanwhile, Egypt said it rejects any move to set up safe corridors for refugees fleeing the enclave, Egyptian security sources said Wednesday.

Gaza, a tiny coastal strip of land wedged between Israel in the north and east, and Egypt to the southwest, is home to some 2.3 million people who have been living under a blockade since the Palestinian resistance group Hamas took control there in 2007.

Egypt has long restricted the flow of Gazans onto its territory, even during the fiercest conflicts.

Cairo, a frequent mediator between Israel and Palestine, always insists the two sides resolve conflicts within their borders, saying this is the only way Palestinians can secure their right to statehood.

One of the security sources, who asked not to be identified, said Egypt rejected the idea of safe corridors for civilians to protect "the right of Palestinians to hold on to their cause and their land."

Several Arab states still have camps for Palestinian refugees who are descendants of those who were forced out of their homes when Israel was created in 1948. Palestine and other Arab states have said a final peace deal needs to include the right of those refugees to return, a move Israel has always rejected.