Israel has launched air strikes against militant targets in the Gaza Strip, after rockets were fired from the territory towards Jerusalem.
The Hamas-run Palestinian health ministry in Gaza said 20 people, including children, had died in the strikes.
In Jerusalem, the rocket fire caused Israel’s parliament to be evacuated as sirens sounded.
Tensions between Israel and the Palestinians spiralled in recent days.
Gaza’s Hamas rulers had threatened to strike after hundreds of Palestinians were injured in clashes with Israeli police in Jerusalem yesterday.
Israel said it had killed at least three Hamas militants.
Hamas sources told reporters a commander of the Izzedine al-Qassam Brigades, Mohammed Abdullah Fayyad, had been killed.
Israel’s Prime Minister, Benjamin Netanyahu, said Hamas had “crossed a red line” and that the “current conflict may continue for some time”.