The Hamas-run health ministry in Gaza says at least 90 Palestinians have been killed in an Israeli air strike on a designated humanitarian area.
About 300 people have been injured, according to the health ministry’s statement, in an attack which Israel says targeted senior Hamas leader Mohammed Deif and his deputy Rafa Salama.
In a news conference on Saturday evening, Israeli Prime Minister Benjamin Netanyahu said there was “no certainty” that either of them had been killed.
The strike hit the al-Mawasi area near Khan Younis, which the Israeli military has designated as a humanitarian zone.
An eyewitness in al-Mawasi told the BBC that the site of the strike looked like an “earthquake” had hit.
Videos from the area show smouldering wreckage and bloodied casualties being loaded on to stretchers.
People can be seen trying desperately to pick through the rubble of a large crater with their hands.
BBC Verify has analysed footage of the aftermath of the strike, confirming that it took place within an area shown on the Israel Defense Forces (IDF) website as a humanitarian zone.
Mr Netanyahu said he gave the order for the operation to go ahead after being briefed by his general security forces.
He said he wanted to know there were no hostages nearby, the extent of the collateral damage and what kinds of weapons would be used.
During the news conference, he promised to eradicate all of the group’s senior members.
“Either way, we will get to the whole of the leadership of Hamas,” Mr Netanyahu added.
Later Hamas leader Ismail Haniyeh, quoted by the AFP news agency, accused Mr Netanyahu of seeking to block a ceasefire in the Gaza war with “heinous massacres”.
Hamas said the claim that their leaders were targets was “false”.
“It is not the first time Israel claims to target Palestinian leaders, only to be proven false later,” the group said in a statement.
An Israeli military official said the strike took place in an “open area” where there were “no civilians”.
He refused to say whether it was inside a designated safe zone, but said Hamas leaders had “cynically” set up in a civilian area.
The official also said he was unaware of any hostages taken during the 7 October attack on Israel being in the area.
He added that “accurate intelligence” was gathered before the “precision strike”.
One of the doctors at a hospital dealing with the aftermath of the attack has told the BBC it is “one of the black days”.
Speaking to Newshour on the BBC World Service, Dr Mohammed Abu Rayya said the majority of cases coming in were dead, with others suffering from multiple shrapnel wounds.
He said it was like being in “hell”, adding that many of the casualties were civilians, notably women and children.
Footage from the nearby Kuwait field hospital showed scenes of chaos with patients being treated on the floor.
The Nasser medical complex in Khan Younis is “overwhelmed” and no longer able to function, said British charity Medical Aid for Palestinians.
Mohammed Deif, the head of Hamas’s military wing the al-Qassam Brigades, is one of Israel’s most wanted men.
He has near-mythical status in Gaza after escaping capture and surviving several assassination attempts, including one in 2002 when he lost an eye.
He was imprisoned by Israeli authorities in 1989, after which he formed the Brigades with the aim of capturing Israeli soldiers.
Israel accuses him of planning and supervising bus bombings which killed tens of Israelis in 1996, and of involvement in the capture and killing of three Israeli soldiers in the mid-1990s.
It is thought he was one of the masterminds behind the 7 October Hamas attack, when about 1,200 Israelis and foreigners – mostly civilians – were killed and 251 others were taken back to Gaza as hostages.
It led to the major Israeli military operation in Gaza which has killed more than 38,400 Palestinians, according to the Hamas-run health ministry.
A Hamas official, cited by Reuters, called Saturday’s attack a “grave escalation” that showed Israel was not interested in reaching a ceasefire agreement.
The ceasefire negotiations being held in Qatar and Egypt ended on Friday without success, the BBC understands.
Gaza’s Hamas-run civil defence agency said that in a separate incident, 17 people had been killed in an Israeli strike west of Gaza.
The attack is said to have targeted a prayer hall in the Shati refugee camp to the west of Gaza City. The Israeli military has not yet commented on the claim.