President Bashar al-Assad
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 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, Syria's most populous city, on Friday.
The US state department says it fears a massacre by Syrian government forces.
The pro-government al-Watan newspaper warned that the "mother of all battles" was about to start.
"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".
UK Foreign Secretary William Hague said the people of Aleppo were threatened with a potential massacre.
"This utterly unacceptable escalation of the conflict could lead to a devastating loss of civilian life and a humanitarian disaster," he said.
He urged all permanent members of the UN Security Council - including Russia and China, who have vetoed three resolutions - to condemn the actions.
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.
An activist based in Fardos in the city told the BBC at least 15 people had died on Friday morning during the military's bombardment of a building.
"We have medical supplies but no doctors or equipment to treat the injured. The situation feels hopeless," said the activist, identified only as Ramy.
"The people of Aleppo are not coping with this crisis. They are dying. It is a massacre. People can leave their homes and move around the city but who would really want to take the risk of being shot or bombed?"
He insisted that activists would continue to resist the government forces.