Tel Aviv/Gaza  - Israel launched its long-expected ground invasion of the Gaza Strip Saturday night, as infantry and armour, backed by air support, moved into the north of the salient after an artillery barrage lasting hours.

An Israeli military spokesman said the ground operation was the second stage of the Israeli offensive which began last Saturday, and was intended to destroy Hamas installations in the area of operations, and to lower the number of rockets being fired at Israel.

Hamas militants opened fire on the Israelis as they advanced into the Strip. There were no immediate reports of casualties.

The ground operation had been expected for days, after Israel called up thousands of reservists and deployed them along the border, while the air offensive continued unabated.

Israel Radio reported Saturday night that the government has authorized the call-up of more reserve troops.

The Israel Air Force (IAF) attacked 40 targets in the Gaza Strip Saturday, and militants in the enclave continued launching rockets at the Jewish state.

For the first time since the offensive began one week ago, Israeli guns positioned along the border opened up in the late afternoon in what turned out to be a "softening up" prior to the ground incursion.

The artillery barrage continued for several hours. It was unclear whether it was an artillery shell or an airstrike which hit a mosque in the north of the Strip in the late afternoon, killing 11 Palestinians and wounding 50 others, 24 of them seriously.

It was not the first mosque hit in the Israeli offensive; the Israeli military has said militant Palestinians used mosques as weapons depots and even as rocket-launching bases.

Military officials in Tel Aviv however had no immediate comment on the strike on the mosque Saturday afternoon.

One of the IAF strikes in the early hours of the morning killed a top Hamas commander, the third high-ranking leader of the Islamist group to be killed in the eight days of the Israeli offensive.

Zakaria al-Jamal was hit when planes attacked the vehicle in which he was driving.

The Israeli military said in a statement that Jamal was a battalion commander in the Hamas military wing, and the head of the movement's rocket-launching squads in Gaza City.

Israel also attacked a car carrying another top Hamas commander, Muhammad Maaruf, and a companion in the early afternoon. Their fate was unclear.

Other targets hit in the IAF strikes included a college, which the military statement said had been used as a rocket-launching base, and the homes of two Hamas militants. One home had been used as a weapons depot and the other as a meeting place to plan attacks, the statement said.

Four Palestinians were also killed in a strike on Rafah, in the south, bringing the day's death toll to at least 19.

Reports from the Gaza Strip also said Israeli aircraft attacked two bridges in the centre of the salient, making movement between the south of the enclave and Gaza City, in the north, harder.

Gaza militants, for their part, continued to launch rockets and mortar shells at Israel, with around 15 attacks, including six with long-range Grad missiles, reported.

One Grad hit a building in the port city of Ashdod, lightly injuring two people and causing damage to the building.

Another rocket fired earlier Saturday morning struck a house in the coastal city of Ashkelon, sparking a fire in the building's yard. No injuries were reported.

Israel launched its "Operation Cast Lead" one week ago, in response to a week of heavy rocket barrages on the Jewish state out of the Gaza Strip following the end of a nervous six-month truce between Israel and Hamas leaders in the salient.

Some 461 Palestinians have been killed in the hundreds of Israeli strikes, and around 2,300 wounded.

The approximately 450 Palestinian rockets and mortars launched since the start of the operation have resulted in four Israelis killed, three of them civilians, and dozens more wounded.

Although the United Nations Relief and Works Agency (UNRWA) has said that at least a quarter of the Palestinian casualties are civilian, exiled Hamas leader Khaled Mashaal said his organization's losses have been "minimal" and that "the resistance and its infrastructure are fine."

Mashaal warned that Israel that its soldiers would face "a dark fate" if Israel sent ground forces into the Strip.

"You, soldiers of the occupiers, have to realize that a dark fate of death, injury and captivity will wait for you" if Israel decides to invade the Gaza Strip," Mishaal said from Damascus. (dpa)