Israel-Gaza updates: Biden speaks with Netanyahu about hostages' release, Gaza humanitarian assistance

More than a week after Hamas terrorists rained thousands of rockets down on Israel and infiltrated the country by air, sea and land in an unprecedented surprise attack, the region stands on the brink of war -- with fears of a wider conflict and thousands dead on both sides of the Israel-Gaza border.

The conflict was touched off by the Oct. 7 sneak attack, which included thousands of armed Hamas fighters breaching a border security fence and indiscriminately gunning down Israeli civilians and soldiers taken off guard.

Israel has responded with a barrage of airstrikes that have decimated the Gaza Strip.

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.

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."