Greece and Iran have reportedly granted Russia permission to fly over their territory when supplying aid to Syria.
The Interfax news agency quoted a Russian Embassy official in Tehran as saying on September 9 that Iran approved all of Moscow's requests on flights delivering humanitarian aid to Syria.
Separately, a Russian Embassy official in Athens told TASS that Greece granted Russia the right to use its airspace for humanitarian flights to Syria on August 31.
Greece said this week that the United States had asked it to close its airspace to Russian aid flights to Syria because of concerns that Moscow might be building up military forces to support Syrian President Bashar al-Assad.
Bulgaria’s Foreign Minister Daniel Mitov said on September 9 that Sofia could allow Syria-bound Russian planes to cross its airspace if Moscow permits it to inspect the cargo at a Bulgarian airport.
Meanwhile, Russia’s Foreign Ministry has acknowledged that Russian military experts were present in Syria.
Ministry spokeswoman Maria Zakharova said on September 9 that the experts were assisting with Russian arms deliveries to Syria, which Moscow says are aimed at combating terrorism.
She also said U.S. requests to Greece and Bulgaria to close their airspace for Russian flights to Syria amount to "international boorishness."
The comments come a day after Washington said it was "concerned by reports that Russia may have deployed additional military personnel and aircraft to Syria."
White House spokesman Josh Earnest also warned that a Russian military buildup in Syria could lead to a "confrontation" with the U.S.-led coalition carrying out air strikes against the Islamic State (IS) extremist group.