U.S. officials have been quoted as saying that Russia has started flying drone aircraft on surveillance missions in Syria.
The two unidentified officials, who spoke to the Reuters news agency on September 21, could not say how many drones were involved in such missions or the scope of the flights.
Meanwhile, the AFP news agency cited two American officials as saying Russia had deployed 28 fighter and bomber aircraft at an airfield in the western Syrian province of Latakia.
One of the unidentified officials said there were also some 20 Russian combat and transport helicopters at the base.
The Pentagon said it could not discuss intelligence matters.
The West has been concerned over Russia's military buildup in Syria, suspecting that the intention is to support the government of President Bashar al-Assad.
Moscow says its aim is to help Syria, a longtime ally, fight the Islamic State (IS) extremist group.
The United States has also warned that Russia's military deployment in Syria could pose a threat to the U.S.-led international coalition fighting IS militants.
U.S. and Russian defense chiefs agreed last week to explore ways to avoid accidental interactions.