An Israeli operation against Palestinian fighters has killed 28 people, medics said Wednesday, and wounded more than 150, in the deadliest day of violence in the coastal strip since 2012.

Air strikes which began in Gaza on Tuesday killed 24 people, including two women and five children, and Israeli forces killed four Hamas fighters who staged a beachfront assault on an army base just north of the besieged Strip.

The first casualties early Wednesday morning were six members of the same family, emergences services spokesman Ashraf al-Qudra said, when an air strike hit their home in Beit Hanoun in northern Gaza.

The home belonged to a commander of the Al-Quds Brigades, the armed wing of Palestinian Islamic Jihad, Qudra said, and the strike killed that commander.

It also killed five of his family members, including his parents, another woman and two children, Qudra said.

Another strike early Wednesday on the southern city of Rafah killed a young man.

In the worst strike on Tuesday, 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 and an eight-year-old were among the dead and that at least another 25 people were wounded.

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.

Before that, 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 fighters, 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 fighter.

Meanwhile on Tuesday, 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 fighters, 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 but 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.