Bulgaria has denied a Russian request to use its airspace for supply flights to Syria due to serious concerns about the cargo on the planes, the Bulgarian foreign ministry's spokeswoman said on Tuesday.

"The Bulgarian foreign ministry has refused flights over Bulgaria of Russian military transport planes en route to Syria," the spokeswoman told Reuters. She said the decision was taken in the past few days.

"We have enough information that makes us have serious doubts about the cargo of the planes, which is the reason for the refusal," she added.

The Russian foreign ministry declined immediate comment.