Palestinian medical officials say an Israeli airstrike has hit a U.N. school in the northern Gaza Strip, the second deadly Israeli airstrike to hit a U.N. school on Tuesday.
Israel's military said its shelling was a response to mortar fire from within the school, pressing its assertion that Hamas militants are using civilians as cover. Two residents of the area who spoke to the Associated Press by telephone said they saw a handful of militants firing mortar shells from a street near the school.
Israel has accused Hamas of using schools, mosques and residential areas for cover and staging grounds for terrorist attacks.
Earlier Tuesday there was an attack that killed three at a school in the Gaza city itself.
U.N. and Palestinian medics said 40 Palestinians were killed in the attack, including children, women and elderly people.
Palestinian health official Said Joudeh told the Associated Press the school was turned into a shelter for people displaced by Israel's offensive against Hamas militants.
Also on Tuesday, Israeli tanks blasted their way into the southern Gaza city of Khan Yunis for the first time, Palestinian witnesses said, as heavy fighting raged on the northern edges of Gaza City itself.
Shells slammed into Gaza and ground forces edged closer to major population centers, taking more civilian lives after Israel declined mounting international calls for an immediate cease-fire on Monday. A Palestinian rocket attack wounded an Israeli infant.
In fighting that raged early in the morning, at least 18 people were killed in shelling up and down the Gaza Strip 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, witnesses said sounds of fighting could be heard from around the new Israeli positions. Israel already has encircled Gaza City, the area's biggest city.
Israel launched its offensive on Dec. 27 in a bid 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 U.N. figures. Nine Israelis have died since the operation began.
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.
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 allowing delivery of vital supplies.
"There's nowhere safe in Gaza. Everyone here is terrorized and traumatized," said John Ging, the top U.N. official in Gaza, blaming the international community for allowing the violence to continue.
"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."
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 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. One landed in the town of Gadera, about 25 miles from the Gaza border, lightly wounding a 3-month-old infant, police said.
Israeli forces have cut the main Gaza highway in several places, compartmentalizing the strip into the north, south and Gaza City itself and preventing movement between them. Israel also has taken over high-rise buildings in Gaza City and attacked destroyed dozens of smuggling tunnels — Hamas' main lifeline — along the Egyptian border.
Late Monday, a paratroops 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.
The international community, on the other hand, has been more cautious, defending Israel's right to defend itself but expressing concern about the rising civilian death toll.
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.
She said international relief organizations have complained that there is a serious problem distributing aid in Gaza.
The EU delegation was one of a flurry of diplomatic efforts to forge a cease-fire. French President Nicolas Sarkozy left Israel after 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 in New York, 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 crossing points 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 a shell fired by an Israeli ship hit their house on the Gaza shore, hospital officials said. Local residents said the gunboat apparently fired at a group of militants next to the house who were preparing to ambush advancing Israeli troops. Two of the militants were killed in the blast.
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.
Holmes said Gaza was in the grip of an "increasingly alarming" humanitarian crisis and was running low on clean water, power, food, medicine and other basics.
On Tuesday the Israeli military said three soldiers were killed and 24 wounded in a friendly fire incident when an Israeli tank shelled a building in which they had taken cover Monday night during fighting outside Gaza City. The military said a colonel who commanded an infantry brigade was among the wounded.
In a separate friendly fire incident, also Monday, a paratroops officer was killed in northern Gaza, the army said. In all, six soldiers have been killed since the offensive began.
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 sparked anger 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.
Hamas is listed as a terrorist organization by the United States and many other Western nations. From 2000 to 2004, Hamas was responsible for killing nearly 400 Israelis and wounding more than 2,000 in 425 attacks, according to the Israeli Ministry of Foreign Affairs.
From 2001 through May 2008, Hamas launched more than 3,000 Qassam rockets and 2,500 mortar attacks against Israeli targets.
The Associated Press contributed to this report.