The victims include six children and three women as well as several commanders of the Palestinian Islamic Jihad (PIJ).
Palestinian factions in Gaza continued to fire rockets in retaliation from the besieged coastal enclave into Israel, killing one person on Thursday.
Amid mediation efforts by Egypt, neither side seemed ready to douse the worst flare-up since August.
“We are at the height of a campaign, both offensive and defensive,” Israeli Prime Minister Benjamin Netanyahu said in a videotaped statement issued during a visit to an air base.
“Whoever comes to harm us – his blood is forfeit,” he added.
Egypt announced it was trying to secure a truce but so far, its efforts have proved futile.
Cairo, which hosted senior PIJ official Mohammad al-Hindi for talks, was circumspect about prospects for a ceasefire.
“Egypt’s efforts to calm things down and resume the political process have not yet borne fruit,” Egyptian Foreign Minister Sameh Shoukry told reporters.
Meeting Jordanian, French and German counterparts in Berlin, Shoukry urged “peace-sponsoring countries to intervene and stop the attacks” and said Israel must “stop the unilateral measures that aim to destroy the future of the Palestinian state”.
The initial Israeli air attacks on Tuesday that set off the exchange of fire killed three senior PIJ fighters and at least 10 civilians, most of them women and children.
The deaths of Ali Ghali and Ahmed Abu Daqqa brought to five the number of senior figures from PIJ killed since Israel began attacking Gaza early on Tuesday.
More than 90 people have been wounded in the attacks that destroyed five buildings and damaged more than 300 apartments, said Salama Marouf, chairman of the media office for the Hamas group that governs Gaza.
Israel has kept crossings for the movement of people and goods closed since Tuesday, blocking travel entirely, even for urgent humanitarian needs, and preventing patients from accessing medical treatment not available in Gaza, human rights groups have said.
Gaza Strip is home to more than two million people, making it one of the most densely populated areas in the world. It has been under an Israeli air, land and sea blockade since 2007.