A defaced poster of Syria's President Bashar
A defaced poster of Syria's President BasharReuters

The United States said on Thursday it appeared that forces loyal to Syrian President Bashar al-Assad were "lining up" for a massacre in the city of Aleppo, but again ruled out military intervention in the conflict.

Reuters quoted the U.S. State Department as having 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,” she added.

Nuland said, however, that despite U.S. concern over the violence, the Obama administration did not contemplate ending its self-imposed ban on direct military help.

“We do not believe that pouring more fuel on the fire is going to save lives,” she said, according to Reuters. “The route out of this is not more violence ... the route out of this is an end to the violence and a beginning to a true political transition process.”

She dismissed comparisons between Aleppo and the Libyan city of Benghazi, the cradle of the uprising against Muammar Qaddafi, where warnings of a threatened massacre by Libyan government forces in March 2011 prompted the UN Security Council to authorize military action to protect civilians.

“There are a vast number of differences,” Nuland said, citing lack of a UN mandate, more difficult terrain and the absence of a direct call for help by a unified opposition.

“The vast majority of the Syrians want the violence to end. They don't want increased violence,” she added. “The kind of groundswell call for external support that we've seen elsewhere is not there.”

Aleppo has become the main battleground in Syria's 16-month popular uprising as government forces on Thursday intensified their firepower in response to rebel gains in Syria's largest city.

Aleppo, Syria's principal economic center of 2.5 million, is of strategic importance to the rebels due to its position near the Turkish border, where weapons can be acquired and foreign fighters are said to be joining the opposition.

Fighting is said to be concentrated in three neighborhoods. Forces loyal to Syrian president Bashar al-Assad have allegedly used warplanes and artillery to bomb rebels armed with rifles.

Meanwhile on Thursday, Russia charged that calls for Assad to step down are blocking efforts to end 16 months of unrest in the country.

Russian Foreign Minister Sergei Lavrov warned such calls -- made by the United States, several European and Arab governments and Turkey -- were fanning the flames of violence.

He also reiterated Moscow's claim that support for Syrian rebel groups was tantamount to backing terrorism.

Israel has reportedly reinforced its Syrian frontier as the fighting intensifies. Israeli troops have reportedly been put on “very high” alert.