Working in strife-torn Gaza has exposed United Nations official Christopher Gunness to the terror of a life lived in the constant shadow of war.

But on Wednesday, when artillery shells rained down on a UN school in a refugee camp sheltering evacuees, killing children as they slept next to their parents on the floor, the unsurmountable horror of the situation suddenly overwhelmed him.

Mr Gunness, a senior director for the United Nations Relief and Works Agency for Palestine Refugees in the Near East, broke down sobbing during a live television interview with Al-Jazeera, as he described how shells had struck the Jabalia Elementary Girls School in the Jabalia refugee camp in Gaza, killing about 20 people and wounding at least 100.

The shelling came despite 17 warnings about the position of the shelter to the Israel Defence Force (IDF) to ensure it would be spared, Mr Gunness, from Britain, claimed.

"The rights of Palestinians, even their children, are wholesale denied, and it's appalling," Mr Gunness said during the interview, barely keeping his composure.

As the interviewer thanked him for his time, Mr Gunness coughed, rapidly blinked, and his chin quivered. He managed to say "My pleasure", before putting his hands over his face and completely breaking down in tears at his desk.

Another person then emerged from behind the camera to comfort Mr Gunness, before the camera panned away.

"It's OK," the person could be heard saying, trying to console him.

Mr Gunness, a former BBC reporter who has also worked as a spokesman for the UN in the former Yugoslavia, has given many interviews during the latest conflict, and has managed to keep his composure until now.

He later tweeted that he and his organisation had reached breaking point.

UNRWA is overwhelmed in #Gaza we have reached breaking point, our staff are being killed our shelters overflowing. Where will it end? RT — Chris Gunness (@ChrisGunness) July 30, 2014

Witnesses and UN officials said Wednesday's attack was the latest in a series of strikes on UN facilities that are supposed to be safe zones in the 24-day-old battle between Israel and Hamas and other militants.

More than 3300 Palestinian families were sheltering in the school after fleeing military operations in Gaza.

 

VIDEO: The aftermath of the school bombing: Ruth Pollard reports from Gaza.

During the assault on the school, one shell blew out the front wall of a classroom, another tore a large hole in the ceiling of a second-floor classroom across the courtyard, and another hit a small building near the school gates. Bloodied pillows and blankets were scattered over the school’s courtyard, while four donkeys lay dead at the school's gate.

Reuters reported that UN Secretary-General Ban Ki-moon condemned the attack as outrageous and unjustifiable.

"It demands accountability and justice," he said. "Nothing is more shameful than attacking sleeping children."

Lieutenant Colonel Peter Lerner, from the Israeli military, told The New York Times that no UN facility had been targeted during the operation.

An IDF spokeswoman told Fairfax Media that its initial investigation revealed militants had “fired mortars at IDF soldiers from the vicinity of the UNRWA school in Jabalia. In response, the soldiers fired towards the origins of fire and this incident is still being reviewed."

On Tuesday, the UNRWA said that it had found weapons were being stored in schools that were closed for the summer.

“We condemn the group or groups who endangered civilians by placing these munitions in our school,” Mr Gunness said then.

“This is yet another flagrant violation of the neutrality of our premises. We call on all the warring parties to respect the inviolability of UN property.”