The Pentagon acknowledged Monday that Russia has been building up a military base in Syria.
Pentagon spokesperson Captain Jeff Davis said that the U.S. has seen a "continued steady flow" of equipment and people for the past week and a half.
Davis would not say how many people or give specifics on what types of equipment are moving in to the Latakia area, but he did say that the flow "suggests the establishment of a forward-operating air base."
He said that the U.S. is concerned about "deconfliction" of airspace, but would not comment on whether the coalition has invited Russia to join the air campaign against ISIS in Syria. Davis did say that the U.S. would "welcome Russia's participation in the global anti-ISIL efforts."
In recent days, U.S. defense officials began calling the Russian military activity in Syria a military build-up and are comparing it to Russian activity last year in Crimea.
Two defense officials say that the Russians have brought in dozens of armored vehicles and ground support vehicles, and that they have now also brought in artillery.
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.