Israel launched air strikes against targets in the Gaza Strip after a rocket fired from the Palestinian territory exploded in the city of Ashkelon.

The air strikes targeted a training camp in Gaza City used by the Hamas Islamist group and smuggling tunnels along the enclave's southern border with Egypt. Another air strike targeted an empty caravan in the central Gaza Strip.

Palestinian medical workers said four people were lightly wounded from debris in Gaza City. The Israeli army had no immediate comment.

Palestinian militants in Gaza fired a rocket into Ashkelon on Israel's Mediterranean coast on earlier on Friday, blowing out the windows of an apartment block and damaging parked cars in a residential area of the city.

No one was injured in the blast. But the attack ended over a year of calm for the city closest to the enclave ruled by Hamas.

A police spokesman said the rocket was a 122 mm Chinese-made Grad, with a heavier payload and greater range than the crude, homemade rockets Gaza militants were launching daily until Israel's three-week military offensive into Gaza 18 months ago.

Ashkelon, with a population of 125,000, lies on the coast about 12 km (7 miles) north of the Gaza Strip.

Israeli media reported that Israel had lodged a protest with the United Nations for an attack targeting civilians in violation of international law.