"Yes, they have increased aircraft and there are certain kinds of aircraft there which, depending on what Russia's decision is about its long-term intentions, could raise questions," Kerry said.
"But for the moment, it is the judgment of our military and most experts that the level and type represents basically force protection for their deployment to an airbase, given the fact that it is in an area of conflict."
Syrian military officials from Bashar al-Assad's government say that new weapons delivered to them by Moscow have allowed them to step up their operations against rebel fighters.
It is not clear to what extent Russian forces will be directly involved in fighting, but Washington is concerned the build-up will embolden Assad to hold out against calls to step down in favor of a transitional government.
"Support for Assad is support for a Shiite minority government that dropped barrel bombs on its own people, that has gassed its own people, that has committed war crimes, starved and tortured its own people," Kerry said.