GAZA STRIP (AP) — A massive blast rocked a Gaza City hospital packed with wounded and other Palestinians seeking shelter Tuesday, killing hundreds of people, the Hamas-run Health Ministry said. Hamas blamed an Israeli airstrike, while the Israeli military said the hospital was hit by a rocket misfired by Palestinian militants.

The Gaza Health Ministry said at least 500 people were killed. Video that The Associated Press confirmed was from the hospital showed fire engulfing the building and the hospital’s grounds strewn with torn bodies, many of them young children. Around them in the grass were blankets, school backpacks and other belongings.

Outrage over what many believed was an Israeli strike flared across the region, a day before President Joe Biden was due to arrive to show support for Israel and try to prevent the war from spreading. The carnage unfolded as the U.S. tried to convince Israel to allow the delivery of supplies to desperate civilians, aid groups and hospitals in the tiny Gaza Strip, which has been under a complete siege since Hamas’ deadly rampage in southern Israel last week.

In the south, continued strikes killed dozens of civilians and at least one senior Hamas figure Tuesday in attacks it says are targeted at militants. U.S. officials worked to convince Israel to allow delivery of supplies to desperate civilians, aid groups and hospitals after days of failed hopes for an opening in the siege.

With Israel barring entry of water, fuel and food into Gaza since Hamas’ brutal attack last week, U.S. Secretary of State Antony Blinken secured an agreement with Israeli Prime Minister Benjamin Netanyahu to discuss creation of a mechanism for delivering aid to the territory’s 2.3 million people. U.S. officials said the gain might appear modest, but stressed that it was a significant step forward.

Still, as of late Tuesday, there was no deal in place. A top Israeli official said Tuesday his country was demanding guarantees that Hamas militants would not seize any aid deliveries. Tzahi Hanegbi, head of Israel’s National Security Council, suggested entry of aid also depended on the return of hostages held by Hamas.

“The return of the hostages, which is sacred in our eyes, is a key component in any humanitarian efforts,” he told reporters, without elaborating whether Israel was demanding the release of all of the roughly 200 people Hamas abducted before allowing supplies in.

U.S. President Joe Biden prepared to head to the region as he and other world leaders tried to prevent the war from sparking a broader regional conflict. Violence flared Tuesday along Israel’s border with Lebanon, where Iranian-backed Hezbollah militants operate.

With tens of thousands of troops massed along the border, Israel has been expected to launch a ground invasion into Gaza — but plans remained uncertain.

“We are preparing for the next stages of war,” military spokesman Lt. Col. Richard Hecht said. “We haven’t said what they will be. Everybody’s talking about a ground offensive. It might be something different.”

In Gaza, dozens of injured were rushed to hospitals after heavy attacks outside the southern cities of Rafah and Khan Younis, residents reported. Bassem Naim, a senior Hamas official and former health minister, reported 27 people were killed in Rafah and 30 in Khan Younis.

An Associated Press reporter saw around 50 bodies brought to Nasser Hospital in Khan Younis. Family members came to claim the bodies, wrapped in white bedsheets, some soaked in blood.

An airstrike in Deir al Balah reduced a house to rubble, killing a man and 11 women and children inside and in a neighboring house, some of whom had evacuated from Gaza City. Witnesses said there was no warning before the strike.

Shelling from Israeli tanks hit a U.N. school in central Gaza where 4,000 Palestinians had taken refuge, killing six people and wounding dozens, the United Nations Palestinian refugee agency said. At least 24 U.N. installations have been hit the past week, killing at least 14 of the agency’s staff.