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."

It’s not money that people in the Gaza Strip need right now, but rather food, water, medicine and other basic necessities, according to a humanitarian worker there.

"If we have money, we can’t buy anything. There's nothing to buy in the stores, it’s empty shelves," Omar Alnajjar, project manager for nonprofit Save Your Future, told ABC News in a telephone interview from the city of Rafah in southern Gaza.

As Israeli forces continue to bomb "terror targets" in Gaza in retaliation for an Oct. 7 attack carried out by the Hamas militant group that governs the territory, an estimated 1 million people are displaced there, including over 520,000 people staying in "emergency shelters in increasingly dire conditions," according to the United Nations Office for the Coordination of Humanitarian Affairs.

That's why it's crucial Egypt allows humanitarian aid convoys parked at its northwestern border with Gaza to cross into the war-torn territory and deliver the lifesaving supplies as soon as possible, Alnajjar said. But as of Friday afternoon, the Egyptian-controlled Rafah border crossing remains closed.

"People are just waiting," he added.

-ABC News' Ibtissem Guenfoud, James Longman and Morgan Winsor