Gaza City, Gaza - President Joe Biden will visit Israel Wednesday in a show of "ironclad" support as concern is growing that the escalating war on Gaza could spill over into a wider Middle East conflict.

The trip will come 12 days after the Palestinian militants burst through Israel's heavily fortified Gaza border, shooting, stabbing and burning to death more than 1,400 people, most of them civilians.

Shell-shocked Israel has responded with withering air strikes that have killed thousands of people, also mainly civilians. It has also imposed a crippling siege on Gaza and deployed tens of thousands of troops to the border in preparation for a full-scale ground offensive.

Israelis are still reeling from the worst attack in the country's 75-year history, which has brought a mass mobilization of reservists and the evacuation of about 500,000 people from areas near Gaza and Lebanon.

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US Secretary of State Antony Blinken, back in Israel after a whistlestop regional tour, said Biden's visit would be a statement of "solidarity with Israel" and an "ironclad commitment to its security".

Washington has already sent two aircraft carrier strike groups to the eastern Mediterranean "to deter hostile actions against Israel".

Israel's arch foe Iran has repeatedly warned against a Gaza invasion and Monday raised the specter of a possible "preemptive action" against Israel.

Even after 11 days of atrocities in the Israel-Gaza war, the bombing of Al-Ahli Arabi hospital has caused a level of outrage unlike anything that's come before. Gaza's health ministry says thousands were in the building at the time of the airstrike, with at least 500 confirmed dead.

Palestinians in the West Bank are protesting against their government, led by Mahmoud Abbas, who is seen as weak and ineffective in his response to Israeli abuses.

Meanwhile, thousands are marching in the streets of Turkey, Jordan, Tunisia, and Iran as fury grows at the thousands of deaths in Gaza. That pressure is now starting to tell at the political level. Abbas has officially canceled his planned summit with President Joe Biden, as has Jordan's King Abdullah, who has been a steadfast US ally.

UN ambassadors from Arab countries put up a united front in a joint press conference not long ago, laying the blame for the catastrophe squarely on Israel. The IDF has insisted it has intelligence that indicates the attack was perpetrated by Palestinian Islamic Jihad.

Biden, meanwhile, has boarded his flight to Israel and is expected to arrive Wednesday morning local time.

The Israel Defense Forces are blaming the Gaza hospital bombing on Palestinian Islamic Jihad, saying "intelligence from sources" indicate the strike was caused by a failed rocket launch.

IDF Spokesperson Daniel Hagari offered no other evidence in a short statement.

The Al-Ahli Arabi Baptist Hospital was hit by Israeli assaults before, with four medical staff injured on Saturday.

The Al-Ahli Arabi Baptist Hospital in Gaza City has been bombed, according to multiple reports, with a preliminary death toll of over 500 people.

Al Jazeera is reporting that hundreds of bodies are buried under the rubble of the building, and that no prior warning was given.

Iran's supreme leader Ayatollah Ali Khamenei warned Tuesday that "no one can stop" forces opposed to Israel if it keeps up its bombardment of Gaza.

"If the crimes of the Zionist (Israeli) regime continue, Muslims and resistance forces will become impatient, and no one can stop them," said Khamenei, who has the final say in major state policies in Iran.

"No one should expect" that certain parties like Iran can "prevent the resistance forces" from taking action, he added.

"Regarding the situation in Gaza, we all have a responsibility to react. We must react," Khamenei insisted, without detailing what a reaction may entail.

The death toll in Gaza has passed 3,000, according to the territory's health ministry, with well over half of that number being women and children.

At least six people were killed and dozens were injured on Tuesday when a UN school was shelled, a statement by the United Nations Relief and Works Agency for Palestine Refugees in the Near East (UNRWA) said.

UNRWA Commissioner General Philippe Lazzarini said a UNRWA school was hit in al-Maghazi refugee camp, in Gaza’s central area.

"Dozens were injured – including UNRWA staff – and severe structural damage was caused to the school. The numbers are likely to be higher," Lazzarini said.

"This is outrageous and it again shows a flagrant disregard for the lives of civilians. No place is safe in Gaza anymore, not even UNRWA facilities," he said.

Amid Israel's relentless bombing campaign, there are at least 199 hostages still held in Gaza by Hamas.

On Monday, the group released a video of one of the captives, a French-Israeli woman named Mia Shem.

Her mother, Keren Shem, made an emotional plea for her safe return, at a Tel Aviv press conference.

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"I ask world leaders that my daughter be returned to us in the state that she is today, as well as the other hostages," she said. "I am begging the world to bring my baby back home."

Uri Rawitz, whose 84-year-old mother, Alva Avraham, was taken during the Hamas attack on the city of Sderot, described the horror of receiving photos showing her on a motorcycle, with two armed men also in the frame.

"Please, return my mother," he says in a video produced by the #BringThemHomeNow grassroots initiative, which raises awareness of those taken.

The Biden administration is showing the first sings that it may start trying to rein Israel's crushing response. Pressure – both international and domestic – is growing to secure some sort of humanitarian relief.

On Monday, antiwar Jewish groups staged a protest in front of the White House demanding an immediate ceasefire, a demand also made by progressive Democrats in Congress in a resolution.

AFP reporters in Gaza said mortuaries were overflowing, and corpses wrapped in white body bags were even being stored in an ice cream truck.

The UN agency for Palestinian refugees says more than one million Palestinians – almost half of Gaza's population of 2.4 million – have fled their homes.

World Health Organization regional director Ahmed Al-Mandhari told AFP that Gaza was barreling towards "real catastrophe".

"There are 24 hours of water, electricity and fuel left," he said.