Stephen O’Brien told reporters during a visit to Damascus that the targeting of non-combatants in the country’s war was “unlawful, unacceptable and must stop”.
He was “particularly appalled” by government air strikes on a rebel-held suburb of the capital on Sunday.
Activists said yesterday that the death toll from the attack on a market in Douma had risen to at least 96.
That would make it one of the bloodiest single incident of the four-year-long conflict, which has so far left more than 250,000 people dead.
A Syrian military source told the Reuters news agency that the air force had targeted the headquarters of the rebel group, Jaysh al-Islam (Army of Islam).
State television also reported that rebels had shelled a government-held district of the northern city of Aleppo yesterday, killing 10 people and wounding 17.
Mr O’Brien called on both sides to protect civilians and respect international humanitarian law at the end of a three-day visit to Syria, his first to the country since becoming the UN’s under-secretary general for humanitarian affairs in May.