UN official for Palestinian refugees Christopher Gunness has broken into tears as he described an attack by Israeli forces on a UN-run school in the Gaza Strip.

"The rights of Palestinians even their children, are wholesale denied... and it's appalling," Gunness says during an Al Jazeera Arabic broadcast, before tears well up in his eyes.

He then attempted to clear his throat and continue but just manages to whisper, "My pleasure" to the interviewer before starting to sob. The camera then pans away while Gunness can still be heard crying.

Gunness spoke after 17 people were killed and 90 wounded in the shelling of the Jabalia Elementary School. At least 1,364 Palestinians, mainly civilians, and over 50 Israelis, mostly soldiers, have been killed in fighting between Israel and Islamist militant group Hamas since 8 July.

He later tweeted that the Relief and Works agency was "overwhelmed" in Gaza.

"We have reached breaking point, our staff are being killed our shelters overflowing. Where will it end?"

He also commented on the crying episode:

Gunness claimed that the precise location of Jabalia Elementary Girls School in Gaza which housed 3,000 displaced Palestinians was communicated to Israeli army 17 times.

Israel insists that its troops did not strike UN facilities intentionally but only responded to Palestinian militants fire. The incident, however, is under review by the Israeli Defence Forces.

UNRWA commissioner-general Pierre Krahenbuhl condemned the attack as a "serious violation of international law".

"Last night, children were killed as they slept next to their parents on the floor of a classroom in a UN designated shelter in Gaza. Children killed in their sleep; this is an affront to all of us, a source of universal shame. Today the world stands disgraced."

Palestinians look at a damaged classroom in a United Nation-run school sheltering Palestinians displaced by an Israeli ground offensive, that witnesses said was hit by Israeli shelling
Palestinians study a damaged classroom in a United Nations-run school used as shelters from Israeli bombardment Reuters