The UN chief says he has received "grisly reports" that Syrian government forces are arbitrarily executing, imprisoning and torturing people in Homs after opposition fighters in the flashpoint city fled.
Ban Ki-moon's comments came as a wounded British photographer, who escaped Homs earlier this week, said he had witnessed Syrian troops carrying out a massacre in the city's Baba Amr district, which has become a symbol of a year-long uprising against President Bashar al-Assad.
"A major assault on Homs took place yesterday," Ban told the UN General Assembly in New York on Friday.
"Civilian losses have clearly been heavy. We continue to receive grisly reports of summary executions, arbitrary detentions and torture."
In some of his toughest criticism of Syria to date, Ban said "this atrocious assault is all the more appalling for having been waged by the government itself, systematically attacking its own people".
Taking to the podium after Ban, Syria's ambassador to the UN, Bashar Jaafari, told the General Assembly that the secretary-general's statements were "slandering" his country.
Ban's remarks included "extremely virulent rhetoric that confines itself to slandering a government based on reports, opinions or hearsay", Jaafari said.
Speaking for 45 minutes, Jaafari said the UN chief's statements "come from the opposition or from people who are abroad or people who are living in countries which are open enemies of Syria".
"It's not a war, it's a massacre, an indiscriminate massacre of men, woman and children"
- Paul Conroy, British photographer
He said reports of a humanitarian crisis were "absolutely false".
During his speech, Jaafari was extremely critical of the UN and other Arab states for their handling of the Syrian crisis.
"The secretary-general is not duly informed," he said, reiterating that the Syrian opposition consisted of "armed terrorist groups".
Earlier on Friday, Ban made a plea for Syria to grant access for aid workers to besieged Syrian towns, describing images of death coming out of them as atrocious.
Ban was speaking after the International Committee of the Red Cross told Syria it was unacceptable that its aid convoy had been prevented from entering Homs where the opposition said Assad's army had committed a massacre.
"It's totally unacceptable, intolerable," he said.
"How as a human being can you bear ... this situation? That really troubles me. I'm deeply sad seeing what's happening."
'It's a massacre'
One activist in Homs told the Reuters news agency: "The Syrian army was holding the convoy up because they want to clean up after what they have done in Baba Amr."
As with other activist reports from Homs, this could not be independently confirmed.
"All men who remained in the neighbourhood aged between 14 and 50 were arrested. We fear they will be massacred. Where is the world?" said one activist.
"The massacres are continuing. They are torturing them and killing [detainees] one by one. They are executing them in batches," another activist, who left Baba Amr on Friday, told Reuters via Skype, a video telephone service.
In-depth coverage of escalating violence across Syria
Many fighters out of the 2,000 who were based in Baba Amr were killed and wounded in the onslaught, they said, adding that a final toll was impossible to give because of the heavy shelling and siege. Hundreds were reported to have fled.
British photographer Paul Conroy, who escaped Homs earlier this week after suffering leg injuries in the shelling, said on Friday 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.
"I'm an ex-artillery gunner so I can kind of follow the patterns - they are systematically moving through neighbourhoods
with munitions that are used for battlefields.
"It's not a war, it's a massacre, an indiscriminate massacre of men, women and children."
Conroy said thousands of people were still in Homs, without power or water, and with hardly any food.
"In years to come we're going to sit and we're going to go 'How did we let this happen under our nose'. There are rooms full of people waiting to die."
The UN says Syrian security forces have killed more than 7,500 civilians since the revolt began last March. Syria's government said in December that "armed terrorists" had killed more than 2,000 soldiers and police during the unrest.