The deadly exchange of fire between Palestinian militants in the Gaza Strip and the Israeli military has escalated significantly, with the UN fearing a “full-scale war”.
More than 1,000 rockets have now been fired by Palestinian militants over 38 hours, Israel said, most at Tel Aviv.
Israel has carried out hundreds of airstrikes, destroying two tower blocks in Gaza on Tuesday and Wednesday.
At least 43 Palestinians and six Israelis have been killed since Monday.
That includes 13 Palestinian children caught up in the conflict.
UN Secretary-General António Guterres said he was “gravely concerned” by the ongoing violence.
The fighting follows weeks of rising tension stoked by violent confrontations between Israeli police and Palestinian protesters culminating in clashes at a site in Jerusalem that is holy to both Muslims and Jews.
Israel’s military says the fighting with Gaza is the severest since 2014.
Video footage from the city showed rockets streaking through the night sky, some exploding as they were hit by Israeli interceptor missiles.
Israeli Defence Minister Benny Gantz said the Israeli strikes were “just the beginning”.
“Terror organisations have been hit hard and will continue to be hit because of their decision to hit Israel,” he said. “We’ll return peace and quiet, for the long term.”
Hamas leader Ismail Haniyeh said in a televised address: “If [Israel] wants to escalate, we are ready for it, and if it wants to stop, we’re also ready.”
The fighting between Israel and Hamas was triggered by days of escalating clashes between Palestinians and Israeli police at a holy hilltop compound in East Jerusalem.
The site is revered by both Muslims, who call it the Haram al-Sharif (Noble Sanctuary), and Jews, for whom it is known as the Temple Mount. Hamas demanded Israel remove police from there and the nearby predominantly Arab district of Sheikh Jarrah, where Palestinian families face eviction by Jewish settlers. Hamas launched rockets when its ultimatum went unheeded.
Palestinian anger had already been stoked by weeks of rising tension in East Jerusalem, inflamed by a series of confrontations with police since the start of the Islamic holy month of Ramadan in mid-April.