An Israeli strike in the southern city of Khan Yunis killed nine children of a pair of married doctors, with the Israeli army saying it was reviewing the reports.
Israel has stepped up its assault on Gaza in recent days, drawing international criticism as well as calls to allow in more supplies after it partially eased a total blockade on aid imposed on March 2.
Civil defence spokesman Mahmud Bassal said the agency had retrieved "the bodies of nine child martyrs, some of them charred, from the home of Dr Hamdi al-Najjar and his wife, Dr Alaa al-Najjar, all of whom were their children".
He added that Hamdi al-Najjar and another son, Adam, were also seriously wounded in the strike on Friday.
A medical source at Nasser Hospital, where Alaa al-Najjar works, gave Adam's age as 10 years old.
Footage of the aftermath released by the civil defence agency showed rescuers recovering badly burned remains from the damaged home.
Asked about the incident, the Israeli military said it had "struck a number of suspects who were identified operating from a structure" near its troops.
"The Khan Yunis area is a dangerous warzone," it added.
"The claim regarding harm to uninvolved civilians is under review."
The Israeli army has often claimed that fighters were operating in areas targeted by their forces, without producing verified evidence.
The children's funeral took place at Nasser Hospital, AFP footage showed.
Muneer Alboursh, director general of the health ministry in Hamas-run Gaza, said on X that the strike happened shortly after Hamdi Al-Najjar returned home from driving his wife to work.
"This is the reality our medical staff in Gaza endure. Words fall short in describing the pain," he said, accusing Israel of "wiping out entire families".
Bassal told AFP that Israeli strikes killed at least 15 people on Saturday across Gaza.
He said the dead included a couple who were killed with their two young children in a pre-dawn strike on a house in the Amal quarter of Khan Yunis.
To the west of the city, at least five people were killed by a drone strike on a crowd of people that had gathered to wait for aid trucks, he added.
At Nasser Hospital, tearful mourners gathered Saturday around white-shrouded bodies outside.
"Suddenly, a missile from an F-16 destroyed the entire house, and all of them were civilians -- my sister, her husband and their children," said Wissam Al-Madhoun.
"We found them lying in the street. What did this child do to (Israeli Prime Minister Benjamin) Netanyahu?"
In a statement, the military said that over the past day the air force had struck more than 100 targets across the territory.
Israel resumed its assault on Gaza on March 18, ending a two-month ceasefire.
Gaza's health ministry said Saturday that at least 3,747 people had been killed in the territory since then, taking the war's overall toll to 53,901, mostly civilians.