Military flights traveling from Moscow to Syria have been refused passage over EU member Bulgaria. Sofia has expressed doubts over the cargo, amid fears that Russia is boosting military support for the Assad regime.
Bulgaria said on Tuesday it was denying Russia's request to fly its planes to Syria over its territory.
"The Bulgarian Foreign Ministry has refused flights over Bulgaria of Russian military transport planes en route to Syria," Foreign Ministry spokeswoman Betina Zhoteva told the AFP news agency.
She said that her government had information which had spurred doubts about the cargo on the planes.
"The planes were said to carry humanitarian aid but we had information, that we had reason to trust, that the declared cargo was not the real one," Zhoteva said.
There has been no comment from officials in Russia, but Moscow had asked permission from Greece to access the country's airspace for its Syria-bound aid flights, Rodolfos Moronis, spokesman for Greece's interim government, told reporters.
Greece and Bulgaria are members of the Western military alliance NATO.
Russia has a military airbase in Syria and backs Syrian President Bashar al-Assad, who has been facing opposition from Western powers and domestic rebels ever since a revolution against his regime broke out in 2011. The country has been embroiled in civil war between the president's forces and rebels, with jihadist militants of the "Islamic State" worsening the conflict through their expansionist plans in Syrian territory bordering Iraq.
Russian Foreign Minister Sergey Lavrov has said his country "has never concealed that it delivers military equipment to official Syrian authorities with the aim of combating terrorism."