The Palestinian Ma'an news agency said 20 people were injured in a second day of strikes.
Ynetnews.com reported rockets fired at southern Israel from Gaza wounded eight people Friday, one of them seriously.
Since Friday, more than 60 rockets have been fired at Israel from Gaza and an Israeli defense system designed to target rockets headed toward populated areas intercepted 25 of 27 rockets, Haaretz reported.
Israeli warplanes struck six targets in Gaza identified as two weapons manufacturing sites and two rocket-launching sites in northern Gaza, a weapons manufacturing site in central Gaza and what Haaretz called a "terror activity center" in southern Gaza.
Of the 12 Palestinians killed, most were Islamic Jihad operatives, Haaretz said.