The residents of Israel's Tel Aviv appear not to be panicking by the first rockets to hit the city in 20 years, which Palestinian group Hamas keeps firing in reaction to an Israeli offensive in Gaza.

The new rocket strikes on Tel Aviv are the first since Saddam Hussein's Scud missiles 20 years ago and Palestinian suicide bomb attacks a decade later. On Friday, sirens wailed and another explosion was heard, the second in 24 hours. No one was hurt.

In Gaza, at least 14 Palestinian civilians have been killed by Israeli strikes since the conflict escalated on Wednesday. Three Israeli civilians were killed by a rocket attack on Thursday in a town well to the south of Tel Aviv, where rocketing has become commonplace.

Thousands of rockets have been fired at southern Israel since Hamas took over the enclave in 2007. Israel invaded Gaza in the winter of 2008-2009 to stop the rocket fire, a war in which 1,400 Palestinians and 13 Israelis were killed. Rocketing stopped for a few years but resumed again in 2012.

This is the first time that Islamist Hamas militants in Gaza have fired rockets with sufficient range to reach Tel Aviv and its outlying dormitory cities. None of the gleaming office towers that reflect the setting sun has been scratched.

Hamas, which also targeted Jerusalem on Friday, is gambling with a game-changing move.

Israeli Defense Minister Ehud Barak said the militants would be made to pay. A lethal strike on Tel Aviv would probably trigger an Israeli invasion.

Tel Aviv residents who remember the Gulf War 20 years ago say the rockets still seem less dangerous than Saddam's Scuds, when Israelis wore gas masks in case the Iraqi leader topped his missiles with chemical warheads.

A nationwide early-warning system alerts Israelis to incoming rockets. Homes and offices have blast-proof rooms to retreat to within 30 seconds of the wail of sirens.

Those caught outside lie flat on the ground. Drivers get out of their cars and crouch or lie by the roadside until they hear an impact or an all-clear.

The new "Iron Dome" interceptor system is successfully tracking launches from Gaza and knocking out many of those rockets that look as if they might hit residential areas.