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A school in the Jabaliya refugee camp took a direct hit

At least 30 people were killed and 55 injured when Israeli artillery shells landed outside a United Nations-run school in Gaza, UN officials have said.

A number of children were among those who died when the al-Fakhura school in the Jabaliya refugee camp was hit, doctors at nearby hospitals said.

Israel said its soldiers had come under fire from militants inside the school.

Earlier, the International Committee of the Red Cross (ICRC) warned of a "full-blown humanitarian crisis" in Gaza.

Speaking on the 11th day of the Israeli assault, a senior ICRC official, Pierre Kraehenbuhl, said life in Gaza had become intolerable.

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Palestinian medical sources say up to 600 people have been killed since the attacks began, and Mr Kraehenbuhl said much more needed to be done to protect civilians.

Some 70 Palestinians and five Israeli soldiers were killed on Tuesday.

One soldier was killed in an exchange of fire with militants in Gaza City, while four others were killed by shellfire from their own tanks earlier in the day, Israeli military officials said.

Israel says its offensive is stopping militants firing rockets, but at least five hit southern Israel on Tuesday, with one reaching the town of Gedera, about 40km (25 miles) from Gaza, and injuring a baby.

Three Israeli civilians and one soldier have been killed by rocket fire from the Gaza Strip since the offensive began 10 days ago.

'Mortar fire'

The UN aid agency in Gaza, Unrwa, said three artillery shells had landed close to the al-Fakhura school on Tuesday afternoon, spraying shrapnel on people both inside and outside the building.

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About 350 people had sought refuge at the school in effort to escape the fighting between Israeli soldiers and militants on the outskirts of the refugee camp, to the east of Gaza City.

Television footage showed bodies scattered on the ground amid pools of *lo**.

Palestinian doctors at nearby hospitals had earlier said that at least 40 people had been killed. The Kamal Adwan hospital in Beit Lahiya said 30 people had died there. A further 10 people died at the al-Shifa hospital in Gaza City, doctors said.

The UN officials said they regularly provided the Israeli military with exact co-ordinates of their facilities, and that the school was in a built-up area.

An Israeli military spokesman said that, according to initial checks, its soldiers had come under mortar fire from militants inside the school.

"In response, the forces fired a number of mortar rounds into the area," he told the Reuters news agency.

Israeli Foreign Minister Tzipi Livni said militants regularly hid among civilians inside such facilities or used them to launch attacks.

"According to our values and the instructions that we have in the army, the targets are Hamas targets. We try to avoid any kind of civilian casualties," she told reporters.

This was the second Israeli strike on a UN-run school in a day. Earlier, at least three Palestinians were killed when a school was hit in the Shati camp, UN officials said.

After the first attack, the director of operations for Unrwa, John Ging, said the conditions in Gaza were "horrific".

"Nowhere is safe for civilians here in Gaza at the moment. They are fleeing their homes and they are right to do it when you look at the casualty numbers."

"It's very, very dangerous, and even the 14,000 who have sought refuge in our schools and shelters, they are not safe either."

Mr Ging said international leaders had a responsibility to act to protect civilians.

"You cannot conduct huge military operations in such densely-populated places without killing hundreds and injuring thousands of civilians," he added.

Meanwhile, Israeli forces have pushed further south on the Gaza Strip and clashed with militants near Gaza City.

Skirmishes were also reported on the edges of the Deir al-Balah and Bureij refugee camps in central Gaza, and witnesses said Israeli tanks and infantry were advancing on Khan Younis.

Information about what is happening inside Gaza is limited as Israel has barred foreign reporters from entering.

'Time against us'

Diplomatic efforts to try to end the violence are gathering pace.

French President Nicolas Sarkozy said he had asked his Syrian counterpart, Bashar Assad, to help convince Hamas to co-operate with efforts to end the Israeli offensive. Syria is regarded as a main backer of Hamas.