Israeli strikes on Gaza Tuesday killed 17 people and wounded more than 100, the emergency services said, as the military began an aerial campaign against Hamas members in the Strip.
Meanwhile, in a commando-style raid, four members from Hamas's armed wing emerged from the sea and attacked an Israeli army base north of Gaza, but were all killed by Israeli forces.
In the worst strike, a missile slammed into a house in the southern city of Khan Yunis after people had reportedly formed a human shield to protect it, killing seven people.
Emergency services spokesman Ashraf al-Qudra told AFP two teenagers were among the dead and that at least another 25 people were wounded.
An eight-year-old child wounded in the strike later died of his injuries.
Witnesses said an Israeli drone fired a warning flare, prompting relatives and neighbours to gather at the house as a human shield and that, shortly afterwards, an F-16 warplane fired a missile that levelled the building.
In response, Hamas said "all Israelis" would be potential targets for retaliation.
Shortly afterwards, two Palestinians were killed in a strike in Shejaiya, east of Gaza City, Qudra said.
The blast shook buildings in the city, and Hamas-run Al-Aqsa TV replayed what it said was a video of the strike, which sent a tall cloud of debris rising into the air.
A further three separate air strikes killed three people, including a teenager.
One hit a tuktuk, or motorised rickshaw, in Beit Lahiya in the north of Gaza, killing a "young man," and one west of Gaza City killed a 16-year-old boy, Qudra said.
The third, a while later, hit Deit al-Balah in central Gaza killing a Palestinian man, he said, without identifying him.
Earlier, three people were killed when a missile struck a car in the centre of Gaza City.
Hamas's Al-Aqsa TV showed gruesome images of charred body parts being loaded onto ambulance stretchers.
Relatives said all of them were Hamas members, identifying one as Mohammed Shaaban, 32, a senior commander in Hamas's armed wing, the Ezzedine Al-Qassam Brigades. He was also head of the group's naval operations.
The army confirmed targeting Shaaban, describing him as "a senior Hamas operative".
In a separate strike near Nusseirat refugee camp in central Gaza, another man was killed, with witnesses saying he was a Hamas militant.
Meanwhile, the army shot dead four men from the Brigades on a beach in Israel after they had infiltrated, not far from the Gaza border.
"A number of terrorists came out of the ocean and attacked the base with Kalashnikov rifles and hand grenades," said army spokesman Lieutenant Colonel Peter Lerner.
Soldiers on the base near kibbutz Zikim, just north of the Gaza Strip, shot two of the militants, aircraft killed a third and the navy killed the fourth, said Lerner.
One soldier was lightly injured.
The Brigades said its men carried out an attack at Zikim although it did not report losses, saying instead there were heavy casualties on the Israeli side.
It said it fired 10 Katyusha rockets at the Israeli base.
The deaths came hours after Israel announced the start of Operation Protective Edge, aimed at stamping out rocket fire on southern Israel and destroying Hamas's military infrastructure.
Lerner said some 120 rockets were fired at Israel, 23 of which were intercepted by the Iron Dome missile defence system, with most of the rest falling on open ground causing no damage or casualties.