Three Russian military cargo jets have landed in Syrian in the recent days.
They have delivered mobile housing units and military equipment.
American officials said that most likely Russian is trying to build its military base and also an air control center in Syria to aid an embattled President Bashar al-Assad.
Previously Bulgaria refused to provide an air space corridor to Russian Syria-bound flights.
Although today Russian officials have confirmed that the airspace has been provided by Iran.
There has been a growing concern over the Russian military buildup in Syria.
Previously Russian officials stated that Russian has never made a secret its cooperation with Syria.
"There have always been Russian equipment there and also the specialists which help to operate and service it." stated Maria Zaharova, and official representative of the Russian Foreign Affairs Ministry.