The former head of the UN observer mission in Syria says it is "only a matter of time" until President Bashar al-Assad's government falls.
But Norwegian Maj Gen Robert Mood, who left Syria last week, said Mr Assad's fall would not necessarily mean an end to the 16-month-old conflict.
Syrian forces renewed their assault on the northern city of Aleppo on Friday.
The US state department says it fears Syrian government forces are preparing to carry out a massacre there.
The pro-government al-Watan newspaper warned that the "mother of all battles" was about to start.
'Reports of atrocities'
"In my opinion it is only a matter of time before a regime that is using such heavy military power and disproportional violence against the civilian population is going to fall," Maj Gen Mood told the Reuters news agency.
Separately, UN High Commissioner for Human Rights Navi Pillay appealed to both sides to spare civilians, citing concerns of "the likelihood of an imminent major confrontation".
Ms Pillay said she had received "as yet unconfirmed reports of atrocities, including extra-judicial killings and shooting of civilians by snipers" in Damascus.
Saying she had also received more reports of opposition fighters torturing or executing prisoners, Ms Pillay stated her belief that "crimes against humanity and war crimes have been, and continue to be, committed in Syria".
The BBC's Wyre Davies, on the Syria-Turkey border, says conditions in Aleppo are reported to be dire.
Thousands of government troops have been drafted in from other areas and are encircling the city, he says.
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