UN Secretary-General Ban Ki-moon said he had received "grisly reports" that Syrian government forces were arbitrarily executing, imprisoning and torturing people in the battle-scarred city of Homs after rebel fighters had fled.
The Baba Amro district of Homs became a symbol of resistance to Syrian President Bashar al-Assad after government troops surrounded it with tanks and artillery and shelled it intensively for weeks, killing and wounding civilians cowering in its ruined buildings.
Rebels withdrew yesterday in a key moment in the year-old uprising against Mr Assad.
An official at Syria's Ministry of Foreign Affairs and Expatriates said the army had "cleansed Baba Amro from the foreign-backed armed groups of terrorists".
Opposition activists said Syrian troops were hunting down and killing insurgents who had stayed to cover their comrades' retreat.
"A major assault on Homs took place yesterday," Mr Ban told the 193-nation UN General Assembly in New York. "Civilian losses have clearly been heavy. We continue to receive grisly reports of summary executions, arbitrary detentions and torture."
Syria's UN Ambassador, Bashar Ja'afari, said Mr Ban's remarks included "extremely virulent rhetoric which confines itself to slandering a government based on reports, opinions or hearsay."
"The secretary-general is not duly informed," he said, reiterating that the Syrian opposition consisted of "armed terrorist groups".
The International Committee of the Red Cross said an aid convoy had reached the bombarded Baba Amro area, under siege for 26 days, but was not allowed to enter.
"It is unacceptable that people who have been in need of emergency assistance for weeks have still not received any help," ICRC President Jakob Kellenberger said in a statement.
Mr Ban urged Damascus to grant immediate access for aid workers, describing the images of death coming out of the country as atrocious.
Syria said it wanted to express its "sadness and sorrow" at the death of US journalist Marie Colvin who was killed during the bombardment on Homs last week.
British photographer Paul Conroy, who escaped Homs after suffering leg injuries in the shelling, said there had been a daily, indiscriminate barrage of the city.
"I've worked in many war zones - I've never seen or been in shelling like this," the Sunday Times photographer told Sky News from a hospital bed in central London.
"It's not a war, it's a massacre, an indiscriminate massacre of men, woman and children."
Conditions in the heavily bombarded district are hellish. TV footage showed heavy snow and freezing weather, with residents lacking electricity or fuel for heating. There is also a shortage of food and medical supplies.
Barely a building has escaped damage from artillery shelling and many are pock-marked with bullet holes.
