Mahmoud Abbas, president of the State of Palestine, is pulling out of his Wednesday meeting with President Joe Biden.

"President Mahmoud Abbas decided to return to the homeland tonight, and he called for an emergency leadership meeting tonight," a senior adviser said Tuesday.

Biden is expected to leave the U.S. Tuesday night to head to Israel and Jordan.

-ABC News' Will Gretsky

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.

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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

"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."

At least 11 Americans are among those killed in Israel, President Joe Biden said in a statement.

He added, "While we are still working to confirm, we believe it is likely that American citizens may be among those being held by Hamas."

"American citizens still remain unaccounted for, and we are working with Israeli officials to obtain more information as to their whereabouts," Biden said. "My heart goes out to every family impacted by the horrible events of the past few days. The pain these families have endured, the enormity of their loss, and the agony of those still awaiting information is unfathomable."

"I have directed my team to work with their Israeli counterparts on every aspect of the hostage crisis, including sharing intelligence and deploying experts from across the United States government to consult with and advise Israeli counterparts on hostage recovery efforts," Biden said.