Bulgaria has rejected a request to allow Russian cargo aircraft said to be carrying humanitarian aid to Syria to overfly its territory, TASS reported on Tuesday.

"We denied permission to let Russian military transport planes cross Bulgarian territory because on the basis of the available information we had reasonable doubts the cargoes were not the declared ones. The Bulgarian Foreign Ministry made that decision on its own," TASS quoted Bulgarian Foreign Ministry spokeswoman Betina Zhoteva as saying.

The corresponding notification was sent in response to Russia’s request over the last weekend, she added.

Also on Tuesday, Reuters reported that Russia plans to use an air route to the east of Greece to transport aid into Syria.

According to a Greek government spokesman, this means the use of Greek airspace for such flights is no longer an issue, the newswire said.

Reuters quoted a Greek Foreign Ministry spokesman as saying on Monday the United States had asked Greece on Saturday to close its airspace for Russian cargo aircraft flying to Syria.

Dmitry Peskov, a spokesman for Russian President Vladimir Putin said on Monday that Moscow had no information on alleged increase of Russian military presence in Syria.

Putin said on Friday it was too early to speak about a possible participation of Russia in military operations against the Islamic State (IS) group in Syria.