Hundreds of people were taking shelter at the Anglican-run al-Ahli Hospital at the time of the explosion, around 7 p.m. local time, according to Palestinian officials.

The blast appears to be far deadlier than a previous attack, which destroyed two floors of the hospital’s cancer center on Oct. 14. That explosion injured four staffers, according to the American Friends of the Episcopal Diocese of Jerusalem.

The shelling appears to be one of many targets bombed by Israel in southern Gaza on Tuesday. Among the dozens of casualties—that included civilians—was at least one senior Hamas official in an assault Israel claimed was aimed at militants.

If early casualty figures are confirmed to be accurate, the attack would be the deadliest airstrike Israel has launched into Gaza through five separate wars, the Associated Press reported.

The Palestinian Civil Defense blasted Israel for what it called an “unprecedented” attack on innocent lives that amounts to genocide.

“The massacre at al-Ahli Arab Hospital is unprecedented in our history,” spokesman Mahmoud Basal told Al Jazeera. “While we’ve witnessed tragedies in past wars and days, what took place tonight is tantamount to a genocide.”

A spokesperson for the Israeli military told the Associated Press he had no details to provide about the explosion.

“We will get the details and update the public,” he told the wire service. “I don’t know to say whether it was an Israeli air strike.”

Footage and photos from the fiery scene showed shattered glass and body parts scattered across the rubble of hospital hallways and rooms.

In the aftermath of the explosion, the Palestinian Authority President Mahmoud Abbas declared three days of mourning and called on Palestinians in the West Bank to observe a commercial strike on Wednesday.

Ziad Shehadah, a doctor in Gaza, told Al Jazeera that the hospital was filled with civilians seeking a safe place amid a seemingly endless barrage of airstrikes from Israel.

“People left their homes thinking they were more dangerous and they move to our schools and hospitals to be safe,” he said. “And in one minute, all of them have been killed at a hospital.”

Shehadah called the attack a “massacre,” projecting that casualty numbers will eventually eclipse 1,000 people.