An "unprecedented catastrophe" is unfolding for civilians in Gaza, according to the United Nations, which is pleading for Islamic leaders to allow humanitarian efforts into the territory to help those trapped there.

The United Nations Relief and Works Agency for Palestine Refugees in the Near East (UNRWA) released a statement Wednesday urging the Council of Foreign Ministers of the Organization of Islamic Cooperation "to firmly and unconditionally support the humanitarian efforts to safeguard civilians in Gaza."

UNRWA Commissioner-General Philippe Lazzarini evoked the "harrowing images" from the Al-Ahli Hospital in Gaza, where nearly 500 people were killed in a blast on Tuesday night, to highlight the plight of the civilians who remain in Gaza. Another air strike struck an UNRWA school sheltering 4,000 displaced people on Tuesday, killing at least six people, Lazzarini said.

"An unprecedented catastrophe is unfolding before our eyes," Lazzarini said. "Gaza is being strangled and the world seems to have lost its humanity."

A humanitarian crisis began in Gaza almost as soon as the retaliatory air strikes from Israel began last week, following the Oct. 7 attack on Israel by the Gaza-based terrorist organization Hamas, experts told ABC News. The territory is "highly dependent" on imports, and with the crossings into Israel and Egypt currently sealed, supplies are running out fast.

Just one week after the latest fighting between Israel and Hamas began, one million people inside Gaza were forced to flee their homes, according to Lazzarini.

Fourteen frontline humanitarian workers from the UNRWA are among the dead in Gaza, Lazzarini noted, adding that since the fighting began, not one shipment of aid has been allowed into Gaza.

A surgical team and 60 tons of humanitarian aid and medical items have been mobilized to the Rafah border crossing from Egypt, into southern Gaza, the International Committee of the Red Cross announced on Tuesday. But the aid was not granted passage on Monday, when the crossing was supposed to be opened.

Images show tractor-trailers filled with supplies and other goods idling on roads leading to Gaza.

Potable water, stocks of food, and other supplies such as hygiene materials and medicine are in short supply in Gaza, Lazzarini said, adding that people are being forced to drink unclean water.

"We are on the brink of a major health and sanitation crisis," Lazzarini said.

A mother named Rana, who is trapped in Gaza with her family, told ABC News that she and her children go to sleep every night in fear that they will never wake up again.

At night, Gaza becomes a "ghost city" in near darkness, with sirens and bomb blasts the only sounds to break the silence, she said.

"We sleep in one room," said Rana, who did not want to provide her last name or location due to safety concerns. "We keep praying."

During remarks from Israel on Wednesday morning, President Joe Biden said that Israel agreed to allow humanitarian assistance to move from Egypt to Gaza. The passage will be subject to inspections and the aid will go to civilians, not Hamas, Biden said.

As of Wednesday evening, it was unclear when the crossings between Gaza and Egypt will open for humanitarian passage.