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The US State Department said that credible reports of tank columns moving on Aleppo along with air strikes by helicopters and fixed-wing aircraft represented a "serious escalation" of the government's efforts to crush an armed rebellion.
"This is the concern: that we will see a massacre in Aleppo and that's what the regime appears to be lining up for," State Department spokeswoman Victoria 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 maintain control, and we are greatly concerned about what they are capable of in Aleppo."
But Nuland said that the United States, which has limited its assistance to the Syrian opposition to non-lethal help such as communications equipment, did not foresee military intervention in the conflict without a mandate from the UN Security Council, where Russia has blocked US-led efforts to rally a stronger response.
"We have to redouble our efforts with like-minded nations outside of the UN system," Nuland said, saying the main objective now was to work with the opposition on plans for an eventual democratic transition.
"When that day comes, we have to have a Syria for all Syrians," Nuland said.