At least 12 Palestinians, including three commanders of the militant group Islamic Jihad, have been killed in Israeli air strikes on the Gaza Strip.

Palestinian health officials said three women and three children were among the dead. Another 20 people were injured.

Israel said it had launched an operation targeting militants who posed an imminent threat to its citizens.

Islamic Jihad has vowed revenge and Gaza-based militants are expected to respond with rocket fire into Israel.

Correspondents say one significant factor will be the extent to which Hamas, which controls the Strip, joins in.

Israel officials are said to be preparing for days of fighting.

Forty Israeli warplanes and helicopters attacked in several waves of pre-dawn air strikes across Gaza, hitting homes and causing panic among residents.

Pictures showed at least two apartments with their fronts ripped away and others damaged.

The Israeli military said that in addition to targeting the three militants its aircraft struck 10 sites used to manufacture weapons and six Islamic Jihad military facilities.

Islamic Jihad's military wing, the al-Quds Brigades, confirmed that three commanders were among those killed.

It identified them as Jihad Shaker al-Ghannam, secretary of the al-Quds Brigades' Military Council; Khalil Salah al-Bahtini, the commander of its Northern Region, and Tariq Muhammad Ezzedine, a leader of its military activities in the occupied West Bank.

"As we mourn our martyr leaders, along with their mujahideen wives and a number of their children, we affirm that the blood of the martyrs will increase our resolve," the al-Quds Brigades said.

"We will not leave our positions, and the resistance will continue, God willing."

Israel's Defence Minister, Yoav Gallant, said: "Any terrorist who harms Israeli citizens will be made to regret it."

Islamic Jihad was behind recent rounds of rocket fire into Israel, some of which was sparked by the death in an Israeli prison of a Palestinian hunger striker.