Israeli Defence Minister Ehud Barak has authorized the call-up of 30,000 army reservists, amid speculation about a possible ground offensive on Gaza.
The developments came after Palestinian militants fired rockets from Gaza 70km (45 miles) north towards Tel Aviv.
There were a number of heavy strikes on Gaza overnight, since Israel killed Ahmed Jabari, the military leader of the Islamist group that controls the territory, on Wednesday.
Israel's army tweeted: "Before action, the IDF disperses warning leaflets calling all civilians to evacuate the area. This happened [on Thursday]."
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.
Read more about it at BBC News