The head of the military wing of the Palestinian Islamist group Hamas, Ahmed Said Khalil al-Jabari, has been killed in an Israeli air strike in Gaza City.He and another senior Hamas official died when the car they were travelling in was hit.
It appears to have been the first step of an Israeli operation against militant groups in the Hamas-run Gaza Strip.
A series of air strikes are now being reported across Gaza.
The Israeli Defence Force (IDF) said it had launched "Operation Cloud Pillar" against Hamas, Islamic Jihad and other militant groups.
In recent days, Israeli officials had said they were considering assassinating senior figures in Hamas following a wave of heavy rocket-fire.
Mr Jabari, who was 46, is the most senior Hamas official to be killed in the Gaza Strip since the major Israeli offensive four years ago.
Israel's Shin Bet security service confirmed he had been targeted, saying he was responsible for "all terrorist activities against Israel from Gaza" in the last decade.
"Jabari was responsible for financing and directing military operations and attacks against Israel. His elimination today is a message to Hamas officials in Gaza that if they continue promoting terrorism against Israel, they will be hurt," it said.
Since Saturday, four Palestinian civilians and three militants, including Mr Jabari, have been killed in Gaza, and dozens injured. Eight Israelis have also been hurt.
Hamas spokesman Taher Anunu told BBC Arabic: "This was a heinous crime that will have repercussions.
"The Palestinian government will call for the trial of those responsible for the crime, and for hitting a civilian area, but the Palestinian factions have the right to respond in a way they see fit."