Russian news agencies quoted Foreign Ministry spokeswoman Maria Zakharova as saying that a US intervention against the Syrian Army "will lead to terrible, tectonic consequences not only on the territory of this country, but also in the region on the whole".
She said regime change in Syria would create a vacuum that would be "quickly filled" by "terrorists of all stripes", AP reported on Saturday.
US-Russian tensions over Syria have escalated since the breakdown of a cease-fire last month, with each side blaming the other for its failure. Syrian government forces backed by Russian warplanes have launched a major onslaught on terrorist-held parts of the northern city of Aleppo.
Russian warplanes struck terrorist-held areas north of Aleppo on Saturday as the army shelled the besieged old quarter in the major offensive.
Russia was reported on Friday to be sending more warplanes to Syria to ramp up its air campaign as the United States said it had not yet given up on finding a diplomatic resolution.
The latest strikes come 10 days into the Syrian government offensive to capture terrorist-held eastern Aleppo and crush the last urban stronghold of the terrorists.
Saturday's airstrikes focused on major supply lines into terrorist-held areas – the Castello Road and Malah district – while fighting raged in the Suleiman al-Halabi neighborhood, the frontline to the north of Aleppo's Old City.
US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov spoke by telephone for a third day on Friday, with Russia's top diplomat saying Moscow was ready to consider more ways to normalize the situation in Aleppo.
But Lavrov criticized Washington's failure to separate moderate armed groups from those the Russians call terrorists, which had allowed forces led by the group formerly known as the Nusra Front to violate the US-Russian truce agreed on Sept. 9.
The United States made clear it would not, at least for now, carry through a threat made on Wednesday to halt the diplomacy if Russia did not take immediate steps to end the violence.
An army source quoted in state media said its forces had made advances, which was denied by terrorists who say they had repelled a new assault.
A news commentary by the state-run Ikhbariyah said "high-level coordination from the air and ground by Syrian and Russian warplanes" had allowed the two allies to "successfully hit locations where terrorist groups had dug in".