Hamas says its military chief has been killed in an Israeli airstrike in Gaza City.
Ahmed al-Jabari was travelling in his vehicle when his car exploded on Wednesday. There are reports his deputy was also killed.
A series of air strikes is now reported across Gaza. The Israeli Defence Force said it has launched "Operation Cloud Pillar" against Hamas, Islamic Jihad and other groups.
Mr Jabari, 46, is the most senior Hamas official to be killed in the Gaza Strip since a major Israeli offensive four years ago.
Israeli officials recently said they were considering assassinating senior figures in Hamas following a wave of heavy rocket-fire.
The BBC reports the 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.
Since Saturday, four Palestinian civilians and three fighters, including Mr Jabari, have been killed in Gaza and dozens wounded.