Russia has called for military-to-military cooperation with the United States to avert "unintended incidents" as it stages navy exercises off the coast of Syria, where U.S. officials believe Moscow is building up forces to protect President Bashar Assad.
The United States is using Syrian air space to lead a campaign of air strikes against Islamic State, and a greater Russian presence raises the prospect of the Cold War superpower foes encountering each other on the battlefield.
Both Moscow and Washington say their enemy is ISIS, whose Islamist fighters control large parts of Syria and Iraq. But Russia supports the government of Assad in Syria, while the United States says his presence makes the situation worse.
In recent days, U.S. officials have described what they say is an increase of Russian equipment and manpower.
President Barack Obama said this would not change U.S. strategy in countering Islamic State fighters, which includes U.S. planes leading an international coalition in airstrikes in Syria.
"But we are going to be engaging Russia to let them know that you can't continue to double-down on a strategy that is doomed to failure," he said at an event with military service members during a visit to Maryland.
In the latest reports, two Western officials and a Russian source told Reuters Moscow is sending advanced SA-22 anti-aircraft missiles to Syria. The system would be operated by Russian troops, rather than Syrians, the Western officials said.
U.S. officials in Washington also said they believed about 200 Russian naval infantry forces were now stationed at an airfield near the Syrian city of Latakia, an Assad stronghold, and that the number had increased in recent days.
At a news conference in Moscow, Foreign Minister Sergei Lavrov said Russia was sending equipment to help Assad fight Islamic State. Russian servicemen were in Syria, he said, primarily to help service that equipment and teach Syrian soldiers how to use it.
A source close to the Russian navy told Reuters a squadron of five Russian ships equipped with guided missiles had set off to conduct maneuvers in Syrian waters.
Lavrov blamed Washington for cutting off direct military-to-military communication between Russia and NATO after the crisis in Ukraine last year. Such contacts were "important for the avoidance of undesired, unintended incidents", Lavrov said.
"We are always in favor of military people talking to each other in a professional way. They understand each other very well," Lavrov said. "If, as (U.S. Secretary of State) John Kerry has said many times, the United States wants those channels frozen, then be our guest."