The Israeli military has urged Israelis living as far as 80 kilometres (50 miles) away from Gaza, or beyond the Tel Aviv area, to open bomb shelters as militants fired dozens of rockets when a truce broke down.
Hamas said after a deadly Israeli air strike that it fired 40 rockets at Israel.
Israeli media reported at least 30 strikes, with one person injured, and that municipalities in the Tel Aviv area had already reopened shelters shut during a 10-day truce period in the five-week-old Gaza war.