GAZA CITY, Gaza — Israeli forces edged closer to Gaza's major population centers today and attacked new sites, including a U.N. school, claiming more civilian lives after ignoring mounting international calls for an immediate cease-fire. A Palestinian rocket attack wounded an Israeli infant.

The United Nations said three civilians were killed in the airstrike late Monday on the courtyard of its school, where hundreds of people from a Gaza City refugee camp had sought shelter from Israel's blistering 11-day offensive against the Hamas militant group.

"There's nowhere safe in Gaza. Everyone here is terrorized and traumatized," said John Ging, the top U.N. official in Gaza.

Israel launched its offensive on Dec. 27 to halt repeated Palestinian rocket attacks on its southern towns. After a weeklong air campaign, Israeli ground forces invaded Gaza over the weekend. More than 500 Palestinians have been killed, including more than 100 civilians, according to United Nations figures. Nine Israelis have died since the operation began.

"I am appealing to political leaders here and in the region and the world to get their act together and stop this," Ging said, speaking at Gaza's largest hospital. "They are responsible for these deaths."

U.N. officials say they provided their location coordinates to Israel's army to ensure that their buildings in Gaza are not targeted. The Israeli army had no comment on the latest strikes, but in the past has accused militants of using schools, mosques and residential neighborhoods to store weapons or launch attacks.

The international Red Cross also was looking into reports that a Red Crescent ambulance station in the northern town of Jebaliya was hit during the night.

In other fighting early today, at least 18 Palestinians were killed in shelling from tanks and naval craft, local hospital officials said. Two of the dead were confirmed as militants.

Tanks rumbled closer to the towns of Khan Younis and Dir el Balah in south and central Gaza but were still several miles outside, witnesses said, adding that the sounds of fighting could be heard from around the new Israeli positions. Israel already has encircled Gaza City, the area's biggest city.