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 Major General 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, 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," Major General 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.
Reports emanating from the Syria-Turkey border, say conditions in Aleppo are dire.
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.
"Activists are prepared to engage in a guerrilla war, from street to street if necessary," he said.
The Red Crescent has suspended some of its operations in Aleppo because of the heavy fighting.
Rebels have been stockpiling ammunition and medical supplies in preparation for the expected assault.
Syrian troops fired from helicopter gunships on south-western neighbourhoods on Friday morning, the Syrian Observatory for Human Rights told the AFP news agency.
A convoy of tanks from Idlib province, near the border with Turkey, arrived in Aleppo overnight and was attacked by rebels, the Observatory said.
The US State Department said the deployment of tanks, helicopter gunships and fixed-winged aircraft around Aleppo suggested an attack was imminent.
But the US would not intervene, said spokeswoman Victoria Nuland, except by continuing to channel non-lethal assistance (such as communications equipment and medical supplies) to the rebels.
A Syrian MP from Aleppo has fled to Turkey, Turkey's state-run Anatolia news agency says.
Ikhlas Badawi, a mother of six, said she was defecting in protest at the "violence against the people".
Meanwhile, another defector, Gen Manaf Tlas, has put himself forward as a possible figure to unite the fractious opposition.
In an interview with a Saudi newspaper, Asharq al-Awsat, he said: "I am discussing with...
people outside Syria to reach a consensus with those inside."
However, some in the opposition regard Gen Tlas - who fled earlier this month - as a compromised figure too close to the government of President Assad.
For its part, Turkey has said it will not tolerate the creation of a Kurdish-run region in northern Syria.
This follows reports that Kurdish rebels in northern Iraq had formed an alliance with a Kurdish party across the border in Syria.
Turkey would strike against "terrorists" in northern Syria, warned Prime Minister Recep Tayyip Erdogan, in the same way it has attacked bases in northern Iraq used by militants linked to the Kurdistan People's Party (PKK).
Turkey is concerned that the creation of a Kurdish authority in the north of Syria could provide a sanctuary to Kurdish rebels fighting for self-rule in Turkey's southeast.