Members of the Syrian Arab Red Crescent unload parcels of medical and humanitarian aid in the Douma neighborhood of Damascus on July 23, 2015. Reuters/Bassam Khabieh

Bulgaria has barred Russian planes flying to Syria from using its airspace, a spokeswoman for the Bulgarian Foreign Ministry told Russian news agency TASS on Tuesday, citing suspicions that the flights, designated as humanitarian aid missions, would not be used for such purposes.

"We denied permission to let Russian military transport planes cross Bulgarian territory because on the basis of the available information, we have reasonable doubts the cargoes are not the declared ones," Bulgarian Foreign Ministry spokesperson Betina Zhoteva said, TASS reported.

Russian planes could go over Iran to deliver aid to Syria, Sputnik News reported, citing Greek newspaper Kathimerini. It could also reroute flights through central Asian countries.

As of Tuesday, the Kremlin had yet to comment on Bulgaria closing its airspace to Russian Syria-bound flights. Dmitry Peskov, the spokesman for Russian President Vladimir Putin, also refused to comment Monday following news that Greece had received a request from the U.S. for Athens to similarly bar Syria-bound Russian planes carrying humanitarian aid from passing through its airspace, TASS reported.

Concerns in Washington are mounting that Russia is building up a military presence in Syria, where it has long supported the embattled President Bashar Assad and also pushed to fight the Islamic State group. Not only has Russia sent a military advance team to Syria, but it has also begun shipping prefabricated housing units and filed flight requests with nearby countries, according to White House officials, the New York Times reported Saturday. Those moves could signify an impending buildup of ground forces.

“We’re also watching their actions very carefully,” John Kirby, State Department spokesman, said, according to the Times. “If these reports are borne out, it would represent a very serious shift in the trajectory of the Syria conflict and call into question any Russian commitment to a peaceful settlement.”

The Kremlin announced in mid-August that it had sent an Il-76 transport plane to the port city of Latakia carrying more than 20 tons of "tinned meat, fish and milk, sugar and essentials, including blankets," the government's report said. The plane was also due to bring back about 90 Russians and other civilians. The governorate of Latakia is a stronghold of Assad that has remained relatively stable in Syria's ongoing four-year civil war, although according to a June 2015 United Nations report, in some villages less than 25 percent of the prewar population remains.