Israeli forces edged closer to Gaza's major population centers on Tuesday and attacked new sites, including a U.N. school, taking 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 on its school, where hundreds of people from a Gaza City refugee camp had gone to seek shelter from Israel's blistering 11-day offensive against the Hamas militant group.

The missile landed in a courtyard late Monday, causing minor damage to the building.

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

"I am appealing to political leaders here and in the region and the world to get their act together and stop this," he 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.

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.

In other fighting early Tuesday, at least 18 Palestinians were killed in shelling from tanks and naval craft, local hospital officials said. Only 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 kilometers (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.

The rising civilian death toll has drawn international condemnations and raised concerns of a looming humanitarian disaster. Many Gazans are without electricity or running water, thousands have been displaced from their homes and residents say that without distribution disrupted, food supplies are running thin.

"This is not a crisis, it's a disaster," said water utility official Munzir Shiblak. "We are not even able to respond to the cry of the people." He said about 800,000 residents in Gaza City and northern parts of the territory had no access to running water from Tuesday.

Israel says it won't stop the assault until its southern towns are freed of the threat of Palestinian rocket fire and it receives international guarantees that Hamas, a militant group backed by Iran and Syria, will not restock its weapons stockpile. It blames Hamas for the civilian casualties, saying the group intentionally seeks cover in crowded residential areas.

"The battle is bitter but unavoidable. We set out on this operation in order to deal Hamas a heavy blow and to alter living conditions in the south of the country and to block smuggling into the Gaza Strip," Israeli Defense Minister Ehud Barak.

The army says it has dealt a harsh blow to Hamas, killing 130 militants in the past two days and greatly reducing the rocket fire. At least 15 rockets were fired Tuesday and one landed in the town of Gadera, about 25 miles (40 kilometers) from the Gaza border, lightly wounding a 3-month-old infant, police said. At the outset of the fighting, militants launched dozens of rockets each day.

Hamas is believed to have 20,000 fighters.

Israeli forces have cut the main Gaza highway in several places, cutting the strip into northern, southern and central sectors and preventing movement between them. Israel also has taken over high-rise buildings in Gaza City and destroyed dozens of smuggling tunnels -- Hamas' main lifeline -- along the Egyptian border.

Late Monday, a paratroop officer and three Israeli infantrymen were killed in two separate friendly fire incidents, the military said. Heavy Israeli casualties could threaten to undermine what so far has been wide public support for the operation.

A high-level European Union delegation met with President Shimon Peres on Tuesday in a futile bid to put an end to the violence. Commissioner Benita Ferraro-Waldner acknowledged Israel's right to self-defense, but said its response was disproportionate.

"We have come to Israel in order to advance the initiative for a humanitarian cease-fire and I will tell you, Mr. President, that you have a serious problem with international advocacy, and that Israel's image is being destroyed," she said, according to a statement from Peres' office.

In Geneva, the international Red Cross said Gaza was in a "full-blown" humanitarian crisis. Its head of operations, Pierre Kraehenbuehl, said the few remaining power supplies could collapse at any moment.

Israeli leaders say there is no humanitarian crisis and that they have allowed the delivery of vital supplies.

The EU delegation was one of a flurry of diplomatic efforts to forge a cease-fire. French President Nicolas Sarkozy left Israel after a day of meetings with leaders.

Europe "wants a cease-fire as quickly as possible," Sarkozy said Monday, urging Israel to halt the offensive, while blaming Hamas for acting "irresponsibly and unpardonably."

Prime Minister Ehud Olmert stressed to Sarkozy that any agreement "must contain at its foundation the total cessation of all arms transfers to Hamas," said Israeli government spokesman Mark Regev.

Regev noted that Hamas used a previous six-month truce to double the range of its rockets. About one-eighth of Israel's 7 million citizens now live in rocket range.

In New York, Arab delegates met with the U.N. Security Council, urging members to adopt a resolution calling for an immediate end to the attacks and a permanent cease-fire.

In Washington, the State Department said the U.S. was pressing for a cease-fire that would include a halt to rocket attacks and an arrangement for reopening crossings on the border with Israel, said spokesman Sean McCormack.

The crossings, used to deliver vital food shipments into Gaza, have been largely closed since Hamas took control of Gaza in June 2007. A third element of a U.S.-backed cease-fire would address the smuggling tunnels used by Hamas.

President George W. Bush emphasized "Israel's desire to protect itself."

"The situation now taking place in Gaza was caused by Hamas," he said.

A top exiled Hamas official in Syria, Moussa Abu Marzouk, rejected the U.S. proposal, telling the AP the U.S. plan seeks to impose "a de facto situation" and encourages Israel to continue its attacks on Gaza.

In Tuesday's fighting, six civilians were killed when an Israeli ship fired a shell at their house on the Gaza shore, hospital officials said. Residents said the gunboat apparently fired at a group of militants next to the house who were preparing to ambush advancing Israeli troops. Two militants died.

Palestinians said Israeli attacks intensified before dawn and at least 10 more civilians were killed when shells hit houses on the edge of Gaza City and in the Jebaliya refugee camp, to the north.

U.N. humanitarian chief John Holmes said the overall Palestinian toll since the opening of the Gaza campaign on Dec. 27 stood at about 500, with about 125 of them civilians.

Israeli forces detained 80 Palestinians -- some of them suspected Hamas members -- and transferred several to Israel for interrogation, said military officials, speaking on condition of anonymity because they were not allowed to release the information.

Israel's operation has angered many across the Arab world and has drawn criticism from countries such as Turkey, Egypt and Jordan, which have ties with Israel and have been intimately involved in Mideast peacemaking.