John Ging, director of operations in Gaza for the U.N. Relief and Works Agency or UNRWA, told reporters at U.N. headquarters by video link from Gaza that three artillery shells landed near the school where 350 people were taking shelter, according to Reuters.
The latest figures in the aftermath of the attack were 30 dead and 55 injured, including at least five critically, Ging said. U.N. staff estimate around 15,000 people have fled to 23 U.N.-run schools, which have turned into makeshifts safety shelters.
An Israeli army spokeswoman told news agencies she was looking into information on the incident at al-Fakhora school in Jebaliya. Another Israeli official, speaking on condition of anonymity, told the Associated Press that Palestinian militants fired on Israeli soldiers from the courtyard of the school, leading to the attack.
Israel launched a broad offensive against Gaza's Hamas rulers on Dec. 27 in a bid to stop rocket fire into southern Israeli towns after a six-month cease-fire expired. The military operation escalated last weekend with Israel's decision to send ground troops into Gaza, driving the fighting closer to population centers in the Palestinian territory.
In fighting on Tuesday, Israeli forces pushed into the southern town of Khan Younis and Dir el Balah in south and central Gaza while also battling Hamas militants on the outskirts of the city of Gaza. Palestinian medical officials said four militants were killed.
In all, the Israeli operation has killed nearly 600 Palestinians, including dozens of civilians, according to U.N. and Palestinian officials.
Sporadic rocket fire has continued to lash southern Israel throughout the conflict. At least five rockets landed in Israel on Tuesday, including one that hit the town of Gadera, some 17 miles from Tel Aviv. Ten Israelis, including three civilians hit by rocket fire, have been killed in the conflict, according to Reuters.
The rising civilian death toll in Gaza has drawn international protests and raised concerns of a looming humanitarian disaster. Many Gazans are without electricity or running water and thousands have been displaced from their homes.
Earlier, the International Committee of the Red Cross warned of a "full-blown humanitarian crisis" in Gaza, the BBC reported.
During Israel's conflict with Lebanon in 2006, the killing of dozens of unarmed Lebanese in an Israeli bombing in the village of Qana drained foreign support for its campaign against Hezbollah guerrillas. Israel said it had not known civilians were in the area.
Meanwhile, world leaders and diplomats continued their efforts to broker a cease-fire.
French President Nicolas Sarkozy told reporters in Lebanon Tuesday that he was returning to Sharm el-Sheikh, Egypt, to meet with Egyptian President Hosni Mubarak to work out the details of a peace plan.
"I'm convinced that there are solutions. We are not far from that. What is needed is simply for one of the players to start for things to go in the right direction," he said during a visit to French U.N. peacekeepers in south Lebanon, the AP reported.
During his tour of the region, Sarkozy also met with Syrian President Bashar Assad in Damascus in a bid to enlist Syria, a key backer of Hamas, in efforts to broker a cease-fire, the New York Times reported. Assad, however, accused Israel of committing a war crime by invading Gaza and said it would pay "the highest price."
Former British Prime Minister Tony Blair, the Middle East envoy of the "Quartet" of major powers sponsoring Israeli-Palestinian peace talks, said Sarkozy, the European Union and the United States were all in agreement that new measures to prevent weapons smuggling into Gaza -- often achieved through the use of a network of underground tunnels -- would be needed to reach a cease-fire.
"What is being talked about is a credible plan to stop the smuggling," Blair told reporters in Jerusalem.
"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 said of the military operation, according to news agencies.
The United States signaled some flexibility in its stance on a truce Tuesday. "We would like an immediate cease-fire, absolutely," U.S. State Department spokesman Sean told reporters after news emerged on Tuesday's strike near the U.N. school in Gaza. "An immediate cease-fire that is durable, sustainable and not time-limited."
Secretary of State Condoleezza Rice planned to travel to New York late Tuesday for a meeting of the U.N. Security Council on the matter.
"We want, obviously, to be constructive," McCormack added. "(Rice) going up there is to signal that we are making every possible diplomatic effort to try to bring about a cease-fire on the terms that we have outlined. We are open to a variety of different formats to bring that about."
Rice is reportedly planning to meet with U.S. allies and several foreign ministers from Arab states to push a three-pronged U.S. strategy to curb rocket attacks, halt the fighting and secure border crossings.