It was an unexpected moment of anguish, never intended to be seen by anyone other than his close colleagues. But a video of a United Nations official sobbing, with his head in his hands, over the plight of children in Gaza has become one of the many memorable images of the war.

Chris Gunness, spokesman for the UN agency for Palestinian refugees, UNRWA, was being interviewed by al-Jazeera Arabic on Wednesday about the shelling of one of the organisation's schools in Gaza, in which at least 15 people died and scores were injured. The school was crammed with families who had fled their homes after warnings from the Israeli military to leave or be bombed. The interview was one of dozens Gunness gave on the incident.

"It was a live interview, and I just about got through it, just about held it together," Gunness, 54, told the Guardian. "But what really makes my heart burst is the suffering of children, and I was so moved by the appalling attack on the school in Jabaliya that I couldn't control myself any longer."

Chris Gunness at the end of the al-Jazeera Arabic interview

Gunness began sobbing after the presenter in Doha had signed off at the end of the live interview. Unknown to him, al-Jazeera's camera kept rolling, and the station later broadcast the entire segment. "For me, it was a moment of private grief. I had no idea it had been broadcast until friends started calling and texting me," he said.

But, he added, "if my tears focus attention on the wholesale denial of human dignity in Gaza, then I have no regrets."

Gunness, a former BBC reporter, has held his Jerusalem-based UNRWA job for more than eight years. In that time he has witnessed three wars in Gaza and has separately waged an energetic media campaign to draw attention to the siege of Yarmouk, a Palestinian refugee camp on the outskirts of Damascus whose inhabitants are trapped in the Syrian civil war.

Following Operation Cast Lead, the three-week war in Gaza in 2008-9, he wrote and performed in a play, I Am A Warehouse, about a storage facility that was repeatedly shelled. It was recently staged in Brighton.

Far from the image of a suited UN bureaucrat, Gunness is passionate about the issue of Palestinian refugees – "truly the dispossessed of the earth", as he described them. He offsets the emotional stresses of his job by running, playing the violin and hosting weekend brunches for friends and journalists in his tiny apartment.

"My tears pale into insignificance compared to those of the people in Gaza, who are suffering intolerably," he said. "But we have now reached a point of such profound tragedy that tears are more eloquent than words."