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."

United Airlines, American Airlines and Delta Airlines announced they will suspend operations to Israel Saturday night following unrest in the region. After two departures scheduled for Saturday, United's future operations will be suspended until conditions allow them to resume, the airline said.

"The safety of our customers and crew is our top priority. We are closely monitoring the situation and we are adjusting flight schedules as required," the carrier told ABC News.

American Airlines said it will temporarily suspended operations to and from Tel Aviv and will continue to monitor the situation, adjusting its operations as needed.

"American Airlines has temporarily suspended operations to and from Tel Aviv, Israel (TLV) [Saturday] and [Sunday], and has issued a travel alert providing additional flexibility to customers whose travel plans are affected. We will continue to monitor the situation with safety and security top of mind and will adjust our operation as needed," American Airlines said in a statement.

Delta Air Lines has canceled scheduled flights in and out of Tel Aviv this weekend. The airline said its working to "safely transport Delta people back to the U.S. and will work with the U.S. government as needed to assist with the repatriation of U.S. citizens who want to return home."

-ABC News' Amanda Maile and Sam Sweeney