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At least 13 Palestinians, including three commanders of the militant group Islamic Jihad, have been killed in Israeli air strikes on the Gaza Strip, BBC News reported.

A Palestinian health official said six women and four 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.

The strikes were the deadliest since three days of fighting between Israel and Islamic Jihad last August.

Islamic Jihad is the biggest militant group next to Hamas in Gaza. It has been responsible for many of the rocket attacks Israel from the territory in recent years and is sworn to Israel's destruction.

There was a serious flare-up last week, as Islamic Jihad and other groups fired several barrages of rockets into Israel over two days following the death in an Israeli prison of a Palestinian hunger striker. The Israeli military carried out air strikes in response.

The latest strikes took place in the early hours of Tuesday morning, when 40 Israeli warplanes and helicopters attacked in several waves across Gaza, hitting homes and causing panic among residents.

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

Islamic Jihad's military wing, the al-Quds Brigades, said three of its commanders were killed, along with their families.