The UN High Commissioner for Human Rights Navi Pillay said more than 5,000 people have been killed during the uprising in Syria.
Pillay was speaking December 12 after a closed briefing with members of the Security Council on the situation in the country.
She added that at least 300 children had been killed and thousands were in detention.
Pillay urged the council to refer Syria to the International Criminal Court for investigation of possible crimes against humanity.
The British ambassador to the UN, Mark Lyall Grant, described Pillay's briefing as "really distressing."
"As one of our colleagues said it was the most horrifying briefing that we've had in the Security Council over the last two years," Grant later told reporters.
"She's catalogued more than 5,000 people killed, a situation that is deteriorating with a military buildup in Homs, tens of thousands of detentions, rapes, torture, violations of abuses right across the system by the Syrian regime.
Russian Ambassador Vitaly Churkin said he too was troubled by Pillay's report but said the Security Council has already taken action on Syria, demanding dialogue to resolve the crisis.
"Unfortunately after that we saw some key members of the international community and some key members of the Security Council switch gears and turn into regime change mode, discouraging dialogue, discouraging dialogue within Syria, discouraging dialogue between Arab League and Syria and we think this is very dangerous, in fact, they make no secret of the fact that they want regime change," Churkin said.
Syrian Ambassador Bashar Ja'Afari said that Pillay had "violated her tasks, her duty, and the honor of her office," in giving a report based on information from Syrian defectors.
He also said Pillay should never have been brought before the council.