He's also condemned Syrian air strikes that have killed nearly 100 people outside the capital during his visit.
The new UN Under-Secretary General for Humanitarian Affairs, Stephen O'Brien, says all sides fighting in the four-year conflict have taken unacceptable actions.
A string of bloody incidents greeted the UN's new humanitarian chief, Stephen O'Brien, as he made his first visit to Syria since taking the role.
He's described the civilian casualties in Syria as a shameful blot on the world's conscience.
"I am absolutely horrified by the total disregard for civilian life by all parties in this conflict. Attacks on civilians are unlawful, unacceptable and must stop. I appeal to every party engaged in violence and fighting to protect civilians and to respect international humanitarian law."
His visit coincided with government strikes in Damascas that killed more than 100 people.
It's one of bloodiest single incidents of the four year-long conflict, which has so far claimed more than 250,000 lives.
Amid the rubble, a man called out to anyone who would listen.
"We appeal to the whole world to stop the injustice on this innocent town. We appeal to the whole world.."
A Syrian military source says the air force had been targeting the headquarters of the rebel group, Jaysh al-Islam.
A new report from rights group Amnesty International accuses Syria's government of committing war crimes against the 163-thousand people living under siege in Douma and other suburbs around Damascus.
The conflict in Syria topped the agenda for Russia's and Iran's Foreign Ministers, as diplomats try to broker some kind of breakthrough in the conflict, now in its fifth year.
Russia's Foreign Minister, Sergei Lavrov, says the conflict must be resolved by Syrians without foreign interference.
"The issues of overcoming the Syrian crisis should be resolved and negotiated with the Syrian government and the Opposition which should represent the whole spectrum of opponents of the Syrian leadership. The transitional steps, reforms, decisions and negotiations should be taken based on the mutual agreement of the government and its opponents."
Elsewhere in the region, Iraq's former Prime Minister, Nouri al-Maliki, may be tried over the loss of the city of Mosul to IS fighters last year.
A report by a parliamentary committee has accused him and other officials of not doing enough to stop the IS group's advances.
Senior army officers could also face court after MPs voted to refer the report to the public prosecutor.
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