Washington made its views known in a telephone call from Secretary of State John Kerry to his Russian counterpart Sergei Lavrov, the State Department said.
"The secretary made clear that if such reports were accurate, these actions could further escalate the conflict, lead to greater loss of innocent life, increase refugee flows and risk confrontation with the anti-ISIL coalition operating in Syria," the State Department said.
In his call with Lavrov, Kerry discussed "US concerns about reports suggesting an imminent enhanced Russian buildup there," the department said.
"The two agreed that discussions on the Syrian conflict would continue in New York later this month," it said.
The New York Times reported that Russia has sent a military advance team to ally Syria and was taking other steps that Washington fears may signal plans to vastly expand its military support for President Bashar al-Assad.
The Times said the moves included the recent transport of prefabricated housing units for hundreds of people to a Syrian airfield and the delivery of a portable air traffic control station there.
Russia's President Vladimir Putin was asked Friday whether Russia was taking part in military operations against the Islamic State group in Syria.
"We are looking at various options but so far what you are talking about is not on the agenda," he said.
Signup to our newsletter and follow us on Facebook and Twitter!
"To say we're ready to do this today -- so far it's premature to talk about this. But we are already giving Syria quite serious help with equipment and training soldiers, with our weapons," RIA Novosti state news agency quoted Putin as saying.
Russia's Foreign Ministry said Saturday's telephone call was made at Kerry's initiative.
It made no mention of US concerns about a possible Russian military buildup, but said the two men discussed "different aspects of the situation in Syria and its environs, as well as the objectives of the fight against IS and other terrorist groups."
The foreign ministry said they spoke of "cooperation" between Moscow and Washington to "support UN efforts aimed at launching a political process in Syria."
Lavrov and Kerry agreed to remain in close contact in pursuing a settlement of the Syrian conflict, which has claimed more than 240,000 lives since March 2011 and driven millions more from their homes.
- Flurry of diplomacy -
In recent weeks, there has been a flurry of diplomatic consultations to try to find a way out of the crisis, including an unprecedented meeting in Doha on August 3 between the top US, Russian and Saudi diplomats.
The Saudi and Iranian foreign ministers were later received separately in Moscow, as were representatives of various more moderate Syrian opposition groups.
Moscow, which has been a bulwark of military and diplomatic support to the Assad regime, is promoting an expanded coalition against IS that includes countries of the region as well as the regular Syrian army.
President Barack Obama for his part received Saudi King Salman on Friday for talks dominated by Syria. They advocate a political solution that would include Assad's departure from power.