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.
In Aleppo itself, a city of two million people and the country's commercial capital, Syrian rebels have begun stockpiling ammunition.
A Syrian security source was quoted by AFP news agency as saying that special forces had begun arriving on the edges of Aleppo in readiness for a "generalised counter-offensive on Friday or Saturday".
A similar account has emerged from the Syrian Observatory for Human Rights, which talks of reinforcements arriving from the main Damascus-to-Aleppo road to the south.
Columns of troops and tanks are also thought to be travelling from the city of Hama, and from the border posts with Turkey in Idlib province.
At least 34 people were killed in the city on Thursday, activists said, as artillery and helicopter gunships attacked rebel targets.
Activist Abu Mohammad al-Halabi told Reuters that so far most of the victims had been civilians, as the rebels were moving around the city to avoid attacks.
"There is lots of internal displacement, and schools have been turned to makeshift shelters that are packed. One shell hitting a school will result in a catastrophe," he said.
The BBC's Ian Pannell, near the city, says thousands of people have already left as fears grow that an intense battle looms.
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, when rebels took control of large parts of the city before being forced to withdraw in the face of a government offensive.
Luke Harding of the Guardian newspaper, 50km (30 miles) west of Aleppo, told the BBC that much of the rest of Aleppo province was in rebel hands but it was an exaggeration to say they were in control of half of the city.
"It's true they're in the southern part of the city and the north-east. Tactically, they're doing guerrilla-style attacks and they can melt away quite quickly."
Foreign journalists operate under heavy restrictions in Syria so claims made by either side are difficult to verify.
In the capital, Damascus, activists said the army had pounded the capital's last rebel-held areas and 20 civilians had died.
Five children and four women were killed, according to the Syrian Observatory for Human Rights, when shells hit the Yalda area. Helicopters also attacked the nearby district of Hajar al-Aswad.
The 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.
Repeated diplomatic attempts to stop the violence have foundered, with the UN Security Council bitterly divided.