MAHMOUD ZAKI/EPA-EFE/REX/Shutterstock US Vice-President Kamala Harris has condemned the loss of civilian life in an Israeli air strike against a school building in Gaza on Saturday.
More than 70 people were killed at the building which sheltered displaced Palestinians, the director of a hospital has told the BBC.
Ms Harris said "far too many" civilians had been killed "yet again"and reiterated calls for a hostage deal and a ceasefire, echoing comments made by the White House.
An Israeli military spokesman said al-Taba’een school "served as an active Hamas and Islamic Jihad military facility", which Hamas denies.
Speaking at a campaign event in Phoenix, Arizona, Ms Harris said Israel had a right to "go after Hamas" but also has "an important responsibility" to avoid civilian casualties.
Saturday's air strike has been criticised by Western and regional powers, with Egypt saying it showed Israel had no desire to reach a ceasefire or end the Gaza war.
Fadl Naeem, head of al-Ahli Hospital where many of the casualties were taken, said around 70 victims were indentified in the hours after the strike - with the remains of many others so badly disfigured that identification was difficult.
Israel's military said it had "precisely struck Hamas terrorists operating within a Hamas command and control centre embedded in the al-Taba'een school".
A statement by the Israel Defense Forces (IDF) and the Israeli Security Agency said "at least 19 Hamas and Islamic Jihad terrorists" were "eliminated" in the attack.
IDF spokesman Rear Adm Daniel Hagari said “various intelligence indications” suggest a “high probability” that the commander of Islamic Jihad’s Central Camps Brigade, Ashraf Juda, was at the Taba’een school when it was struck.
He said it is not yet clear whether the commander was killed in the attack.
The BBC cannot independently verify casualty figures from either side.
The Israeli spokesman said the casualty figures released by Hamas officials "do not align with the information held by the IDF, the precise munitions used, and the accuracy of the strike".
Hamas described the attack as a "horrific crime and a dangerous escalation" in Israel's "war of extermination against the Palestinian people".
US National Security Council spokesman Sean Savett said Hamas had been using schools "as locations to gather and operate out of".
"But we have also said repeatedly and consistently that Israel must take measures to minimise civilian harm," he added.
Israel has attacked several such shelters in Gaza in the past few weeks.
According to the United Nations, 477 out of 564 school buildings in Gaza have been directly hit or damaged as of 6 July, with more than a dozen targeted since.
Al-Taba’een school housed more 1,000 people - having recently received dozens of displaced people from the town of Beit Hanoun, after the Israeli army ordered them to leave their homes.
The building also served as a mosque and the Israeli strike hit during dawn prayers, witnesses said.
Jaafar Taha, a student who lives near the school, told the BBC the sound of the bombing was followed by screaming and noise.
"'Save us, save us,' they were screaming," he said.
"The scene was horrific. There were body parts everywhere and blood covering the walls."
Salim Oweis, spokesman for the UN children's agency, Unicef, told the BBC the attack was "really outrageous".
"All those schools are really packed with civilians, children, mothers and families, who are taking refuge in any empty space whether it's a school or it's a mosque, whatever it is, even in hospital yards."
This strike has again drawn graphic attention to a controversial dynamic of the Gaza war.
Israel claims that Hamas is using civilian infrastructure to plan and carry out attacks, and that is why it has been targeting hospitals and schools - sites protected under international law.
Hamas has consistently denied the accusations.
ReutersHamas-led gunmen killed about 1,200 people in an attack on Israel on 7 October, taking 251 others back to Gaza as hostages.
That attack triggered a massive Israeli military offensive against Gaza and the current war.
More than 39,790 Palestinians have been killed in the Israeli campaign, according to Gaza's Hamas-run health ministry.
Additional reporting by Rushdi Abualouf, Gaza correspondent located in Istanbul, Turkey
