A security source told AFP that Israeli warplanes had struck two bases belonging to Hamas' military wing in northern Gaza last night, causing significant damage.
A medical official at the Shifa Hospital in Gaza City said three people were being treated for injuries, including a 26-year-old man who was in critical condition after being struck in the head with shrapnel.
The Israeli army confirmed the strikes, saying they were in retaliation for a rocket fired a few hours earlier from Gaza, which landed in southern Israel.
That incident caused no casualties or damage, a military spokesperson added.
Israel and Islamists Hamas, who run Gaza, have fought three wars since 2008.
Since the most recent conflict in 2014, a fragile ceasefire has been observed along the largely closed border.
Missiles and rockets are periodically fired into Israel, generally by hardline Islamist groups opposed to Hamas.
But Israel holds Hamas responsible for all rocket fire from Gaza, regardless of who carried it out, and usually retaliates within hours.
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