The US says that it fears Syrian government forces are preparing to carry out a massacre in the country's most populous city, Aleppo.
The US state department said the deployment of tanks, helicopter gunships and fixed-winged aircraft suggested such an attack was imminent.
Spokeswoman Victoria Nuland said it was a "serious escalation" in the conflict.
Syrian rebels in the city have begun stockpiling ammunition and medical supplies in preparation.
Ms Nuland said: "Our hearts are with the people of Aleppo.
And again, this is another desperate attempt by a regime that is going down to try to maintain control."
But she insisted that the US would not intervene other than by providing non-lethal assistance to the rebels who have been trying to topple the government of President Bashar al-Assad for 16 months.
"We do not believe that pouring more fuel on this fire is going to save lives," she said, adding that there had not been "the kind of groundswell call for external support" seen elsewhere.
The US has said it is supporting the opposition with communications equipment and medical supplies.
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.
A prominent defector from the Syrian government, 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 Assad government.