Russia on Tuesday lashed out at Bulgaria over its refusal to allow Russian cargo planes bound for Syria to fly over its territory, saying the move has cast doubts on the nation's independence.
The angry statement comes amid signs of a Russian military build-up in Syria that has raised U.S. concerns. Russian Deputy Foreign Minister Mikhail Bogdanov alleged that Bulgaria and Greece are facing pressure from Washington.
Bulgaria's Foreign Ministry earlier said the country is refusing to allow Russian military transport aircraft to fly through its airspace en route to Syria from Sept. 1-24. It said without elaboration that the reason for the refusal was "incorrect information" about the purpose of the flights and the cargo.
On Monday, Greek Foreign Ministry spokesman Constantinos Koutras said the U.S. has asked Greece to cancel overflight permission for flights headed to Syria. He said Athens is examining the request.
"If they make some restrictive or banning measures at American request, that raises a question about their sovereign right to make decisions about foreign planes crossing its airspace," Bogdanov said, according to the Interfax news agency.
U.S. defense officials, who spoke on condition of anonymity because they were not authorized to publicly discuss the sensitive issue, said the U.S. has seen an increasing number of Russian transport planes seeking diplomatic approval for flights into Syria. They have also seen the movement of some prefabricated housing in Syria, although they haven't seen any troops moving in or becoming involved in actual combat activities, as some media reports suggested.
U.S. Secretary of State John Kerry called his Russian counterpart Sergei Lavrov over the weekend to express concern, warning that if reports about the Russian military build-up in Syria were accurate, it could further escalate the conflict, increase refugee flows and raise the threat of confrontation with the U.S.-led coalition fighting the Islamic State group.
Russia has staunchly backed Syrian President Bashar Assad throughout the nation's 4 1/2-year civil war, providing his regime with weapons and keeping military advisers in Syria.
President Vladimir Putin said again Friday that Russia is providing the Syrian military with weapons and training. Asked if Russia could deploy its troops to Syria to help fight the IS, Putin answered coyly, saying "we are looking at various options."