Israel faced growing international condemnation of its actions in Gaza on Monday after a strike near a UN school sheltering civilians left 10 people dead and sparked widespread outrage.

As Israel's operation in the besieged territory entered its 28th day, the international community appeared to be losing patience with a confrontation the Palestinians say has left more than 1,800 dead in Gaza.

French Foreign Minister Laurent Fabius led the charge, issuing one of the most strongly worded condemnations yet from a Western official.

"How many more deaths will it take to stop... the carnage in Gaza?" Fabius said in a statement.

"Israel's right to security is total, but this right does not justify the killing of children and the slaughter of civilians," Fabius said.

"A political solution is essential... and should in my opinion be imposed by the international community," he said.

Fabius's statement prompted some to praise Western officials for at last taking a tougher line with Israel.

"Finally! This is very, very good. The systematic targeting of civilians and schools must be denounced," said Yves Aubin de la Messuziere, a retired French diplomat and expert on the Middle East.

"We should be brave enough to point the finger of blame, of course at Hamas but also at Israel," he said.

'Very serious' attack

The United Nations condemned Sunday's strike at the school, where around 3,000 homeless Palestinians had been sheltering, with Secretary General Ban Ki-moon calling the attack "a moral outrage and a criminal act".

State Department spokeswoman Jen Psaki said Washington was also "appalled," demanding a "full and prompt" investigation into the strike -- the third time in 10 days a UN school had been hit in the fighting.

The Israeli army acknowledged targeting three Islamic Jihad militants on a motorbike in the "vicinity of an UNRWA school", saying it was investigating.

Israel declared a seven-hour unilateral truce on Monday, after Prime Minister Benjamin Netanyahu said its forces did not intentionally hit civilians and apologised for any harm done to them.

But outrage continued to mount.

Spain roundly criticised the strike, with the foreign ministry in a statement calling it a "very serious" attack.

"Spain reiterates its urgent call for strict respect of International Humanitarian Law and in particular the obligation to avoid hurting the civilian population as well as to respect the inviolability of UN sites," it added.

"Israel's armed forces should intensify their efforts to avoid the loss of innocent lives."

British Prime Minister David Cameron said the United Nations was "right" to condemn the shelling, though he declined to say whether it breached international law.

"The UN has spoken very clearly and I think they're right to speak very clearly," Cameron told the BBC.

"International law is clear that it's completely wrong and illegal to target civilians, if that's what's happened."

Schools 'symbol of tragedy'

UN cultural and education agency UNESCO said at least seven schools run by the United Nations Works and Relief Agency (UNWRA) had been hit since the start of the confrontation.

The agency's chief Irina Bokova urged all sides to ensure that schools were left out of the conflict.

"Schools in Gaza have become the symbol of an unfolding human tragedy," Bokova said in a statement.

"The protection of schools cannot wait. It is essential for the protection of civilians and it is a precondition to start getting the education system back to normalcy as soon as circumstances permit."

Israel's arch-foe Iran meanwhile denounced the "inaction" of the UN Security Council on Gaza, with President Hassan Rouhani accusing the international community of failing "to prevent the crimes against humanity of the Zionist regime".

"The savage aggression by the army of this child-killer regime (Israel), continues with a deliberate policy to commit genocide and massacre civilians and destroy infrastructure, houses, hospitals, schools and mosques," Rouhani said in Tehran.