The military leader of the Palestinian militant group Hamas, Ahmed Jabari, has been assassinated in an Israeli air strike in Gaza.
Israel says the killing marks the start of a broader operation in Gaza.
Hamas said Israel "had opened the gates of hell" and that they have a right to protect themselves from Israel.
Jeremy Bowen, Middle East editor, analysed the political implications of the attacks from both sides.
Speaking to Today presenter John Humphrys, Daniel Taub, Israel's ambassador to the UK, said: "We have to do what any country would do to protect its citizens."
"Sitting and doing nothing is not an option" for Israel, he added.
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