United Nations Secretary-General Ban Ki-moon has expressed concern about reports that Syrian government forces are arbitrarily executing, imprisoning and torturing people in the city of Homs.
The government in Damascus has rejected the accusations.
Mr Ban told the UN General Assembly on Friday that the Syrian government has failed to deliver on its responsibility to protect its people.
A major assault on Homs took place yesterday, he said.
Civilian losses have clearly been heavy. We continue to received grisly reports of summary executions, arbitrary detentions and torture.
This atrocious assault is all the more appalling for having been waged by the government itself, systematically attacking its own people, Mr Ban said.
Earlier Mr Ban made a plea for Damascus to grant immediate access for aid workers to besieged Syrian towns, describing the images of death coming out of the country as atrocious.
Syria has refused to let a Red Cross convoy into the worst parts of Homs, despite saying on Thursday that an aid mission would be allowed.
Mr Ban told the General Assembly he believes the Syrian regime is afraid of letting outside people in.
Journalists return home
Meanwhile, two French journalists who were smuggled out of Homs into Lebanon, have been flown back to a military airport outside Paris.
Edith Bouvier and William Daniels were met on arrival by President Nicolas Sarkozy.
Ms Bouvier was badly wounded in the bombardment of a media centre in Homs, in which two other journalists were killed.
Spanish journalist Javier Espinosa was trapped with them. He says the situation there is very bleak.