The White House on Tuesday warned that a Russian military buildup in Syria could spark a "confrontation" with US-led forces carrying out an air campaign against the Islamic State group.
Amid reports that Russia is readying to step up its support for Bashar Al-Assad's regime by deploying its own military, the White House expressed grave misgivings.
"We've indicated that the United States is concerned by reports that Russia may have deployed additional military personnel and aircraft to Syria," said White House spokesman Josh Earnest.
"These steps could lead to greater loss of life, they could increase refugee flows and risk confrontation with the counter-ISIL coalition that's operating inside of Syria," he said, using an acronym to refer to the Islamic State.
Signup to our newsletter and follow us on Facebook and Twitter!
A US-led coalition has run more than 50,000 sorties as part of more than one-year-old operation to counter Islamic State militants in Iraq and Syria.
It earlier emerged that at least two NATO members have refused the Russian military overflight rights, amid concerns over Moscow's intentions.
Russia insists it is transporting humanitarian aid.
The White House would not be drawn on whether it had asked allies to deny overflight rights, but said Russia's intentions were "difficult to decipher."
"We've certainly made very clear to Russia our views," said Earnest.
"If they were to follow through in providing that kind of support, it could be both destabilizing and counterproductive to the interest of the international community."