Israel killed three senior commanders of the militant Islamic Jihad group in targeted airstrikes early Tuesday, its military said, while Palestinian officials said nine civilians were also killed.
The strikes hit the top floor of an apartment building in Gaza City and a house in the southern city of Rafah.
The Palestinian Health ministry said 20 people were injured, and that ambulances were continuing to evacuate people from the targeted areas.
Israel's military said it targeted three senior commanders of Islamic Jihad, the second most powerful armed group in the enclave, which is controlled by the Islamist militant group, Hamas.
“Any terrorist who harms Israeli citizens will be made to regret it,” said Defence Minister Yoav Gallant.
It identified them as Khalil Bahtini, the Islamic Jihad commander for northern Gaza Strip; Tareq Izzeldeen, the group's intermediary between its Gaza and West Bank members; and Jehad Ghanam, the secretary of the Islamic Jihad's military council.
The military said the three targeted men had been responsible for recent rocket fire toward Israel.
An Israeli military spokesman said the army was looking into reports of civilian deaths but had no immediate comment.
“We're aware of some collateral and we'll learn more as the day goes ahead,” he told a briefing with reporters.
The Palestinian Health ministry said 12 people were killed in total, including the commanders, their wives, several of their children and others nearby.
Hamas leader Ismail Haniyeh warned of repercussions, saying Israel would “pay the price” for the strike.
“Assassinating the leaders with a treacherous operation will not bring security to the occupier, but rather more resistance,” Haniyeh said in a statement.
Airstrikes continued in the early hours, targeting militant training sites.
In the past, Palestinian militant groups in Gaza have retaliated for such targeted killings.
In anticipation of Palestinian rocket attacks in response to the airstrikes, the Israeli military advised residents of communities within 25 miles of Gaza to stay close to designated bomb shelters.
The Iran-backed Islamic Jihad, which is smaller than Gaza’s ruling Hamas group, confirmed the three were among the dead.
The airstrikes came at a time of increased tensions between Israel and militants in the Gaza Strip.
Last week, Gaza militants fired rockets toward southern Israel, and the Israeli military responded with airstrikes following the death of a hunger-striking senior member of the Islamic Jihad in Israeli custody.
The exchange of fire ended with a fragile ceasefire mediated by Egypt, the United Nations and Qatar.
The airstrikes are similar to ones in 2022 in which Israel bombed locations housing commanders of Islamic Jihad group, setting off a three-day blitz that saw the group lose its two top commanders and other dozens of militants.