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."

In remarks with Israeli Prime Minister Benjamin Netanyahu, Secretary of State Antony Blinken said at least 25 Americans had now been confirmed dead in Israel.

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‘We will always be there by your side,’ Blinken tells Netanyahu

Netanyahu described some of the atrocities committed by Hamas.

"Hamas has shown itself to be an enemy of civilization: The massacring of young people in an outdoor music festival, the butchering of entire families, the murder of parents in front of their children and the murder of children in front of their parents. The burning of people alive, the beheadings," he said.

"Hamas should be treated exactly the way ISIS was treated," he said. "They shouldn't be spit out from the community of nations. No leader should meet them. No country should harbor them. And those that do should be sanctioned."

Blinken said he was speaking not only as an American diplomat, but as a Jew, a husband and a father of young children, saying it was impossible for him to look at photos of families killed "and not think of my own children."

"The same time that we've been shocked by the depravity of Hamas, we've also been inspired by the bravery of Israel citizens," he said. "The grandfather who drove over an hour to a kibbutz under siege, armed only with a pistol and rescued his kids and grandkids. The mother who died shielding her teenage son with her body, giving her life to save his -- giving him life for a second time."

Blinken remarked that some U.S. aid had already been delivered, and that more military aid was on his way. He said the administration would work with Congress to meet Israel’s additional needs and that bipartisan support for Israel was "overwhelming."

The secretary called on world leaders to condemn Hamas.

Blinken also reiterated that Israel "has the right -- indeed, the obligation -- to defend itself" but that it was "so important to take every possible precaution to avoid harming civilians."

-ABC News' Shannon Crawford