The Israeli airstrikes come after Gaza militants fired dozens of rockets at southern Israel yesterday, setting off air raid sirens and forcing hundreds of thousands of Israelis to stay indoors.
The military rushed more forces to the border late yesterday and had warned that such an offensive was likely.
A Twitter statement from the Israeli army said the offensive, dubbed "Operation Protective Edge," is intended to "stop the terror Israel's citizens face on a daily basis."
Gaza health official Ashraf Al-Kedra said at least nine Palestinian civilians were brought to a Gaza hospital with light to moderate injuries from the airstrikes, including several who suffered from shock. He said some of the injured Palestinians were treated and released.
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The latest violence came as Israel pressed forward with its investigation of six Jewish youths suspected of abducting and killing a Palestinian teenager, and Israeli leaders sought to calm an emotional debate over whether the country's politically charged atmosphere led to the gruesome crime.
Tensions have been high since three Israeli teenagers kidnapped June 12 in the West Bank were later found dead, followed by last week's slaying of the Palestinian youth in what many suspect was a revenge attack.
Throughout the unrest, Gaza militants have launched more than 200 rockets and mortars into Israel, including close to 100 yesterday alone.
Israel has responded with dozens of airstrikes, but has not been able to halt the attacks. Eight Palestinian militants were killed in fighting yesterday, the highest death toll yet.
The army said at least 70 rockets were fired at Israel from Gaza yesterday, including 40 launched in a single hour after nightfall, setting off air raid sirens up to 80 kilometres from Gaza, the military said.