At least 15 people have been killed - including a baby - after an Israeli shell struck a United Nations-run school in northern Gaza.
A UN official confirmed there were "multiple dead and injured" in the attack on a school in the city of Beit Hanun.
"Precise co-ordinates of the UNRWA shelter in Beit Hanoun had been formally given to the Israeli army" Chris Gunness, a spokesman for the UN Palestinian refugee agency, said on his Twitter account.
Another UN official said the agency had tried to urge people to leave the building earlier, fearing it may be "a potential target".
The Commissioner-General of the United Nations Relief and Works Agency (UNRWA) says three UN teachers were among those killed.
Pierre Krähenbühl said "(There is) no limit to the sorrow and pain we feel. In thoughts with their families".
The facility was being used as a shelter by hundreds of Palestinians fleeing a major Israeli operation in the area. At least 150 people were injured in the blast - the fourth time a UN facility has been hit during the start of the recent conflict.
In a statement, the UN Secretary-General Ban Ki-Moon said he was "appalled" by news of the attack.
"Many have been killed - including women and children - as well as UN staff. Circumstances are still unclear. I strongly condemn this act" he said.
"Throughout the day, our staff had been attempting to arrange a humanitarian pause in the hostilities so that civilians could be evacuated".
"I express my profound condolences to the families of the victims and those of so many hundreds of innocent Gazans who have tragically been killed as a result of the massive Israeli assault" he added.
More than 100,000 Gazans - 5% of the total population - have sought refuge in UNRWA facilities.
This latest development came as the Israeli ambassador to Ireland denied that the killing of Palestinians in Gaza is mass murder by Israel.
More than 700 Palestinians and around 35 Israelis are now believed to have been killed in just over two weeks of violence.
Hamas has insisted it will continue to reject any plan for a ceasefire, unless border crossings in Gaza are opened up.
Israel's ambassador to Ireland, Boaz Modai, told Newstalk Breakfast earlier that the killing of Palestinians in Gaza is not mass murder, it is the result of a war his country did not want:
Boaz Modai said he is a former Israeli soldier himself, and is proud of his army's actions - calling the Israeli army the 'most moral army in the world':
Responding to Presenter Ivan Yates' question about whether Israel is committing war crimes in Gaza, the Ambassador said the UN Human Rights Council should be renamed the 'UN terrorist rights council':
And he asked that anyone who cites international law in relation to possible war crimes should make sure they have read the relevant guidelines:
Meanwhile the government is being condemned for its decision to abstain from a United Nations Human Rights Council vote on whether to investigate Israel's offensive in Gaza.
The 46 member council supported a Palestinian-drafted resolution by 29 votes. The US was the only member to vote against it, while Ireland and 16 other European countries abstained.
Human Rights Commissioner Navi Pillay yesterday said there was a strong possibility Israel was guilty of war crimes, condemning the indiscriminate firing of rockets and mortars by Hamas, into Israel.
Fianna Fáil Senator Averil Power said she was shocked by the government decision to abstain from the vote:
However, Ireland and Germany were among a total of 17 abstentions. They are understood to have abstained in line with all EU members of the Human Rights Council because the draft Palestinian resolution did not pertain to investigate breaches of international humanitarian law on both sides.
Ireland has pledged to support an inquiry mission into actions carried out by both sides.
Campaigners have asked all the major supermarket chains to remove Israeli products from their shelves.
The 'Ireland Palestine Solidarity' campaign has written to grocery stores nationwide, calling for an outright boycott of Israeli produce.
Campaigners have been filling their trolleys with Israeli stock, then taking it to the checkout to complain.
Seren Deeb, whose grandfather and extended family live in Gaza, says these products are contributing to the death of Palestinian children:
In the Middle East, the US has lifted a ban on flights to and from Israel put in place after a rocket attack near Tel Aviv's airport earlier this week.
Many European airlines are still not flying there, on the strong recommendation of the European Aviation Safety Agency.
US Secretary of State John Kerry is in the region as efforts continue to end the conflict.
Mr Kerry said after meeting UN Secretary-General Ban Ki-moon that "some steps forward" have been made, but added "there is still work to be done".
Hamas has meanwhile rejected any truce without the lifting of Israel's eight-year blockade of Gaza.
"We will not accept any initiative that does not lift the blockade on our people and that does not respect their sacrifices," said Hamas leader Khaled Mashaal.
Israel launched a ground offensive in Gaza on July 17 after more than a week of strikes aimed at stopping rocket fire into the country.
More than 700 Palestinians have now died, according to Palestinian medics. 18 people were killed in airstrikes early on Thursday, they said, including a family of six.
The Israeli army said three more soldiers were killed in combat on Wednesday, bringing to 32 the number of soldiers killed since the start of the ground operation.
A Thai farm labourer was also killed when a rocket fired from Gaza struck the greenhouse where he was working in southern Israel.
Listen to the full Israeli Ambassador's interview with Newstalk Breakfast here.