At least 13 Palestinians, including three commanders of the Islamic Jihad militant group, were killed in Israeli airstrikes in the Gaza Strip.
The official representative of the health care of Palestine said that among the dead were six women and four children. Another 20 people were injured.
Israel said it had launched an operation against militants who posed an immediate threat to its citizens.
Islamic Jihad has vowed to retaliate, and the Gaza-based militants are expected to respond by firing rockets at Israel.
Correspondents say one important factor will be the extent to which Hamas, which controls Gaza, joins them.
They say that the Israeli authorities are preparing for many days of fighting.
The strikes were the deadliest since three days of fighting between Israel and Islamic Jihad last August.
Islamic Jihad is the largest militant group after Hamas in Gaza. He is responsible for many rocket attacks on Israel from the territory in recent years and has vowed to destroy Israel.
There was a major flare-up last week when Islamic Jihad and other groups fired several rocket salvos at Israel in the two days following the death of a Palestinian hunger striker in an Israeli prison. In response, the Israeli military carried out airstrikes.
The latest strikes came early Tuesday morning, when 40 Israeli warplanes and helicopters attacked Gaza in several waves, hitting homes and causing panic among residents.
Photos show at least two apartments with torn facades and other damage.
Islamic Jihad’s military wing, the Al-Quds Brigades, said three of its commanders were killed along with their families.
They have been identified as Jihad Shaker al-Ghannam, secretary of the Al-Quds Brigades Military Council; Khalil Salah al-Bakhtini, commander of the Northern Region, and Tariq Muhammad Ezzedine, who he said was “one of the leaders of military operations” in the occupied West Bank.
“As we mourn our martyred leaders along with their mujahideen wives and several of their children, we affirm that the blood of the martyrs will strengthen our resolve,” the Al-Quds Brigades said. “We will not leave our positions, and the resistance will continue, God willing.”
Israeli Defense Minister Yoav Gallant said: “Any terrorist who harms the citizens of Israel will be made to regret it.”
The Israel Defense Forces (IDF) said Bakhtini was Islamic Jihad’s senior operations officer in Gaza and was responsible for rocket fire from Gaza over the past month.
“He posed an immediate threat to the security of Israeli civilians,” it added.
He added that Hannam was a senior member of Islamic Jihad’s missile force, while Ezzedine was responsible for coordinating with the group’s operatives in the West Bank and planning attacks on Israeli civilians.
The Israel Defense Forces said its aircraft also struck 10 facilities used for weapons production and six Islamic Jihad military facilities.
Spokesman Lt. Col. Richard Hecht said the operation “achieved what we wanted,” AFP reported.
When asked about child victims, he answered: “If there were tragic deaths, we will sort it out.”