New rounds of rockets were fired from Gaza after these strikes, and fresh explosions could be heard from Gaza City at around 3:15 am (0115 GMT).

In a statement issued at 02:41 (0041 GMT), the Israeli army confirmed it was "striking in the Gaza Strip".

According to local security sources and witnesses, the first strikes -- at least seven -- hit a training centre of the Ezzedine al-Qassam Brigades, the armed wing of the Palestinian Islamist movement Hamas. The centre is located in the Al-Maghazi refugee camp in the central Gaza Strip.

A second round of air strikes by the Israeli army targeted the al-Qassam Brigades' training centre southwest of Gaza City, according to local security sources.

After the first air strike, an AFP reporter saw two more rockets fired into Israel from the Gaza Strip, and witnesses said several more rockets were fired from various locations.

Gaza, densely populated with 2.3 million people, has been under an Israeli blockade since Hamas took power in 2007.

The Popular Front for the Liberation of Palestine (PFLP), a secular Palestinian armed group, claimed to have carried out "a barrage of rockets... in response to the Zionist aggression on the Gaza Strip".

On the Israeli side, warning sirens sounded in Sderot, a town in southern Israel close to the Gaza Strip, according to the army.