JERUSALEM (CNN) -- Israeli troops began moving into Gaza on Saturday night, intent on taking out Hamas rocket-launching sites, Israel Defense Forces said.

"We have just a short while ago launched the second stage of the operation against Hamas infrastructure," IDF spokeswoman Maj. Avital Leibovich told CNN.

"The goal is to try and take over some of those launching areas that were responsible for the many launches -- the thousands of launches, in fact -- toward the Israeli civilians," she said.

"What this will do is undermine the peace process," said Saeb Erakat, chief Palestinian negotiator for the Palestinian Authority, the government of President Mahmoud Abbas.

Before the incursion, the Israelis used artillery for the first time in the eight-day-old conflict when Israeli tanks fired shells into Gaza from the northern and eastern borders.

Troops had been massed along the border since the IDF began its bombardment of the Hamas-controlled Palestinian territory December 27. Video Watch the conflict unfold on both sides »

Leaflets signed by the commander of the Israeli military were dropped over northern Gaza on Saturday morning, warning residents to "leave the area immediately" to ensure their safety.

"We are trying to be as humane as possible," Leibovich said. "... The civilians are not our target. We are looking only at militants, Hamas militants."

Exiled Hamas leader Khaled Meshaal promised bƖσσdshɛd if Israeli ground troops enter Gaza.

"As for you, the soldiers of the enemy whose leadership are getting you ready to enter Gaza in a ground attack, you need to know that doom will await you and you will be killed, injured and captured," Meshaal, speaking in Syria, said on the Al Jazeera television network Friday.

"If the enemy got into Gaza, our people will fight from one street to the next, from one house to the other, and on every inch of the land." Find out who's who on each side »

An Israeli airstrike hit a mosque in the northern Gaza village of Beit Lahiya on Saturday evening, killing 13 people and wounding 60, according to Palestinian medical sources.

An Israeli attack earlier Saturday killed Azkariah al-Jamal, the commander of Gaza City's rocket-launching squads, Palestinian and IDF sources said.

Another airstrike killed two Hamas militants in Khan Yunis, in southern Gaza, Palestinian officials said.

Leibovich said Hamas militants had launched 20 rockets from Gaza into Israel by late afternoon Saturday. Learn about the background of the conflict »

Rockets set a house on fire in Ashkelon, damaged a kibbutz dining hall and slightly wounded two people in an eight-story building in Ashdod, according to Israeli ambulance services.

"We still see that Hamas is targeting Israeli civilians. Namely, we still did not reach the goal of crippling Hamas' launching capabilities," Leibovich said.

"We still have many more targets to go," she added.

An IDF statement said Israeli missiles fired from jets and ships hit 25 Hamas "outposts, training camps and rocket launching sites" Saturday morning. Video Watch explosions in Gaza »

The houses of two alleged Hamas terror operatives were also hit, the IDF said.

One, it said, was the home of Azadin Hadad, described as the head of the Hamas military group in eastern Gaza City.

The other, in Beit Lahiya, was used to store rocket-launching equipment and was the home of Ismail Renam, who "has a central role in the launching of Grad-type rockets against Israel," the IDF said.

Grad-type rockets have a longer range than the rudimentary Qassam rockets Hamas more commonly uses.
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Israeli officials say four people have been killed and 59 wounded in Hamas rocket attacks in the past week.

Palestinian medical sources said Saturday that at least 435 people have been killed and 2,285 wounded in Gaza.