A senior militant from Islamic Jihad was killed in an Israeli air strike on the Gaza Strip Friday, prompting the militant group to warn Israel has "started a war".
A child was among four people killed in the strikes, the enclave's health ministry said.
The Israeli army said the strikes were part of an operation "against targets in the Palestinian Islamic Jihad" group.
"The IDF (Israeli military) is currently striking in the Gaza Strip. A special situation has been declared on the Israeli home front."
Islamic Jihad said Israel had "started a war against our people".
"We collectively must defend ourselves and our people. We will not allow the enemy's policy of undermining the resistance and our national perseverance," the group said in a statement.
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Palestinians in Gaza City told AFP they witnessed air strikes on an apartment in the central Rimal neighbourhood.
Gaza's health ministry reported four people were killed, "among them a five-year-old girl, targeted by the Israeli occupation".
Islamic Jihad said it was mourning "the great jihadist commander Tayseer al-Jabari 'Abu Mahmud', who was killed in a Zionist assassination in Gaza City", a statement said.
The strikes come four days after Israel closed its two border crossings with Gaza and restricted the movement of Israeli civilians living near the frontier, citing security concerns.
The measures follow the arrest in the occupied West Bank of two senior members of Islamic Jihad, which has a strong presence in Gaza.
Ahead of launching air strikes, Israeli Defence Minister Benny Gantz told Gaza militants: "Your time is up.
"To our enemies, and specifically to the leadership of Hamas and the Palestinian Islamic Jihad, I would like to emphasise: your time is up. The threat will be removed one way or another," he said.
On Friday afternoon, the Israeli military banned large gatherings in communities within 80 kilometres (50 miles) of the Gaza frontier, until Saturday evening.
The measures follow four days of road closures and other restrictions on movement in the border area.
Palestinians including patients and those with Israeli work permits have been prevented from leaving the Gaza Strip since Tuesday, while the goods crossing has also been shut.
Gaza's only power station is at risk of imminent outage due to a lack of fuel supplies through Israel, its manager warned on Thursday.
This week's shutdown of the border area follows a raid by security forces in the northern West Bank district of Jenin.
Israeli forces detained Bassem al-Saadi and another senior member of Islamic Jihad. A 17-year-old member of the group was shot dead by Israeli forces during the raid.