Israel-Gaza updates: IDF preparing for 'wide range of offensive plans'

At least 500 people have been killed in an explosion at Al Ahli Arab Hospital in the middle of Gaza, according to the Gaza Health Ministry.

The hospital served as a shelter for thousands of residents who fled their homes in northern Gaza.

The Israel Defense Forces denied responsibility for the explosion, saying a failed launch by the Palestinian Islamic Jihad caused the blast.

"From the analysis of the operational systems of the IDF, an enemy rocket barrage was carried out towards Israel, which passed in the vicinity of the hospital, when it was hit," IDF said.

The World Health Organization condemned the blast and called for "the immediate active protection of civilians and health care" workers.

This hospital "was one of 20 in the north of the Gaza Strip facing evacuation orders from the Israeli military," the WHO said in a statement. "The order for evacuation has been impossible to carry out given the current insecurity, critical condition of many patients, and lack of ambulances, staff, health system bed capacity, and alternative shelter for those displaced."

Palestinian Authority President Mahmoud Abbas has declared three days of mourning following the attack.

-ABC News' Will Gretsky

Israeli National Security Adviser Tzachi Hanegbi said Tuesday that officials plan to create "safe zones" in southern Gaza as the only places where residents can receive humanitarian aid.

The aid would be "on one condition," Hanegbi said in Hebrew. "If this aid does not reach civilians, but goes to murderers, [the safe zones] will simply not exist."

The Rafah border crossing between Egypt and Gaza has now endured four bombings from Israel, according to a spokesperson for Egypt's Ministry of Foreign Affairs.

The Rafah crossing was shut on Oct. 10 after it was hit by Israeli warplanes on the Palestinian side three times on Oct. 9 and 10.

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Confusion over border crossing between Gaza and Egypt

Egypt has said it will not open its border crossing Rafah unless Israel allows aid to flow into Gaza.

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"We are in constant contact with all U.N. agencies to extract a safe passage for the relief aid," Egyptian Foreign Minister Sameh Shoukry told BBC Radio. "There is not yet any sort of authorization for a safe passage from the other side of the crossing.”

Shoukry also called out Israeli leaders who urged Gaza residents to move south, saying, "Israel’s call for displacement to the South is in full contradiction to international humanitarian law, leaving people with no water, electricity or food."

Six people were killed and dozens were injured when a Gaza school sheltering refugees was hit by Israeli airstrikes, the United Nations Relief and Works Agency said.

At least 4,000 people have taken refuge in the school, UNRWA said.