Russia says it has asked Bulgaria to explain why it rejected Moscow's request to fly over its airspace to deliver supplies to Syria.
Russian Deputy Foreign Minister Mikhail Bogdanov said on September 8 that Bulgarian officials should ask Russia if they have "some doubts" about the flights or the request to use Bulgarian airspace.
The Bulgarian Foreign Ministry said on September 8 that Sofia had "refused flights over Bulgaria of Russian military transport planes en route to Syria" due to "serious doubts about the cargo of the planes."
Bogdanov said if Bulgaria is taking "restrictive or prohibitive measures at the Americans' request" then it raises questions about Sofia's "sovereign right" to make decisions about the usage of its airspace.
Bulgaria is a NATO member and recently opened an alliance command center that was one of six set up in Eastern European countries in response to Russia's annexation of Ukraine's Crimean Peninsula and involvement in the conflict in eastern Ukraine.
A Greek Foreign Ministry official said on September 7 that Athens is "examining" a request from the United States to ban Russian planes from using the country's airspace.
Media reports have led to speculation that Russia is stepping up its military support for Syrian President Bashar al-Assad's government.