REUTERS - At least 23 people were killed across Gaza, Palestinian officials said on Wednesday, by a bombardment Israel said may be just the start of a lengthy offensive against Islamist militants whose rockets struck deeper than ever before into Israel.

Israelis ran for cover late on Tuesday as air-raid sirens sounded in the business capital Tel Aviv and the holy city of Jerusalem, both of which were hit in the Gaza war of November 2012.

Hamas said it also fired a rocket at the major northern city of Haifa, 140 km (88 miles) away, and though this was not confirmed, Israel said a rocket had landed in Hadera, 100 km (60 miles) from Gaza, further than had previously been reached.

While the Israelis reported no casualties, the long-range salvo was celebrated by the outgunned Palestinians as proof of their resolve in hostilities that flared three weeks ago after the abduction and murder of three Jewish seminary students.

The rocket strikes could lead to an Israeli ground invasion, something officials have said is a possible option.