Israeli Defence Minister Ehud Barak has authorised the call-up of 30,000 army reservists, amid speculation about a possible ground offensive on Gaza.
The announcement came after Palestinian militants fired rockets from Gaza 70km (45 miles) north towards Tel Aviv.
Egyptian PM Hisham Qandil is to travel to Gaza on Friday in a show of support.
Fighting has intensified since Israel killed Ahmed Jabari, the military leader of the Islamist group that controls the territory, on Wednesday.
At least 18 Palestinians in Gaza have been killed in Israeli airstrikes, including children, and three Israelis have been killed by Palestinian rocket fire into southern Israel.
'Unacceptable aggression'
By Thursday night, Hamas said it had fired more than 350 rockets from Gaza, of which Israel said 130 had been intercepted by its Iron Dome missile defence system.
But in Tel Aviv, residents took cover after air raid sirens alerted them to a missile threat for the first time there since 1991. One missile landed in an uninhabited area while one is thought to have landed in the sea.
http://www.bbc.co.uk/news/world-middle-east-20351553