Russia has called for Washington to restart direct military-to-military cooperation to avert "unintended incidents" near Syria, at a time when US officials say Moscow is building up forces to protect President Bashar al-Assad's government.
The US is leading a campaign of air strikes against Islamic State (Isil) fighters in Syrian air space, and a greater Russian presence would raise the prospect of the Cold War superpower foes encountering each other on the battlefield.
Both Moscow and Washington say their enemy is Isil. But Russia supports the government of Assad, while the US says his presence makes the situation worse.
In recent days, US officials have described what they say is a build-up of Russian equipment and manpower.
Lebanese sources have told Reuters that at least some Russian troops were now engaged in combat operations in support of Assad's government. Moscow has declined to comment on those reports.
At a news conference, Foreign Minister Sergei Lavrov said Russia was sending equipment to help Assad fight Isil. Russian servicemen were in Syria, he said, primarily to help service that equipment and teach Syrian soldiers how to use it.
Russia was also conducting naval exercises in the eastern Mediterranean.
Mr Lavrov blamed Washington for cutting off direct military-to-military communications between Russia and NATO over the Ukraine crisis, saying such contacts were "important for the avoidance of undesired, unintended incidents".
The reports come at a time when momentum has shifted against Assad's government in Syria's four-year-old civil war.
Western and Arab countries have backed demands from the Syrian opposition that Assad must give way under any negotiated settlement to the war. Assad refuses to go, and so far his enemies have lacked the capability to force him out, leaving the war grinding on for years.