Greece says it is "examining" a request from the United States to ban Russian planes from using Greek airspace to deliver military supplies to Syria.
A Greek Foreign Ministry official said on September 7 that the request by Washington was made two days earlier.
The official said Russia had asked Athens for permission to fly two planes over Greek territory between September 1 and September 24.
Washington is concerned that Moscow plans to increase military support for the government of President Bashar al-Assad.
The New York Times has reported that Russia has sent an advance team to Syria and taken other actions to expand such support.
"The Russian side has never concealed the fact that it is sending military equipment to the Syrian authorities to help them fight terrorism," Russian Foreign Ministry spokeswoman Maria Zakharova told the AFP news agency on September 7.
Russian Duma opposition deputy Dmitry Gudkov said on September 7 that he had sent a letter to Defense Minister Sergei Shoigu inquiring whether Russian troops are fighting in Syria and, if so, whether there had been any casualties.
At least 225,000 people have been killed in fighting in Syria and millions of others forced to flee their homes since Assad's government cracked down on pro-democracy protests in April 2011.