Syrian forces have renewed their assault on the northern city of Aleppo, firing from helicopter gunships on rebel-held areas.
The US state department has said it fears Syrian government forces are preparing to carry out a massacre.
The pro-government al-Watan newspaper has warned that the mother of all battles is about to start.
Rebels in Aleppo, Syria's most populous city, have been stockpiling ammunition and medical supplies in preparation.
Syrian troops fired from helicopter gunships on south-western neighbourhoods of Aleppo, 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.
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 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 state department spokeswoman Victoria Nuland, except by continuing to channel non-lethal assistance such as communications equipment and medical supplies to the rebels.
The BBC's Ian Pannell, near Aleppo, says thousands of people have already left as fears grow that an intense battle looms.
At least 34 people were killed in the city on Thursday, activists said, as artillery and helicopter gunships attacked rebel targets.
Talal al-Mayhani, an activist with connections to the rebel movement in Aleppo, said the battle for the city was likely to play out in a similar way to an earlier battle in the capital Damascus.
There, rebels took control of large parts of the city before being forced to withdraw in the face of a government offensive.
Foreign journalists operate under heavy restrictions in Syria so claims made by either side are difficult to verify.
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 Bashar al-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.
'Lessons from Balkans conflict'
UN Secretary General Ban Ki-Moon said the world must apply the lessons learned from the Bosnian conflict in the 1990s.
He was speaking in Srebrenica, where a UN peacekeeping force failed to stop the killing of more than 7,000 Bosnian Muslim men and boys in 1995.
"I do not want to see any of my successors, after 20 years, visiting Syria, apologising for what we could have done now to protect the civilians in Syria - which we are not doing now," Mr Ban said .
The head of UN peacekeeping operations, Herve Ladsous, defended the decision to reduce the number of observers in Syria.
"We found ourselves with too many people and not enough to do," he said.
Speaking in Damascus, he said there was "no plan B" beyond Joint Special Envoy Kofi Annan's peace plan.
Repeated diplomatic attempts to stop the violence have foundered, with the UN Security Council bitterly divided.
The Syrian government has said its forces are trying to dislodge the "remnants of mercenary terrorist groups".
More than 16,000 people have been killed in Syria since the start of anti-regime protests in March 2011, activists say.