Colonel Steve Warren, a US military spokesman in the region, said Russian intentions remain unclear.
"We have not seen a significant reduction, frankly, in their combat power. Particularly the ground combat power remain static, the air combat power has been slightly reduced, but that's it," he said.
Warren said there were some indications of small units packing up, and eight to 10 Russian aircraft have left the country.
Putin on Monday ordered the Russian defense ministry to begin the withdrawal of the "main part" of its military contingent in Syria.
The first warplanes arrived back in Russia on Tuesday to a hero's welcome.
Signup to our newsletter and follow us on Facebook and Twitter!
But Warren said the US military remains uncertain about Moscow's plans.
"There is a long list of possibilities and rather than getting into each one of them, we're going to continue to focus on fighting ISIL," he said, using an acronym for Islamic State, the extremist group that controls large parts of Syria and Iraq.
The White House, however, said Tuesday that Moscow appeared to be moving forward on its commitment to withdraw forces and US Secretary of State John Kerry said he will travel to Russia next week for a meeting with Putin and Foreign Minister Sergei Lavrov.
Russia intervened militarily in the Syrian civil war September 30 at the request of its close ally, President Bashar al-Assad, deploying about 50 combat aircraft.
It also sent more than 4,000 ground troops, artillery, tanks and about 30 combat helicopters.
The Russians have directed their operations mainly at western-backed anti-government rebels, while a US-led coalition has been waging an air campaign against IS.