Bulgaria said Tuesday it had refused permission for Russian aircraft to cross its airspace last week as concern grows among NATO members that Moscow is boosting military aid to Syria's Bashar al-Assad.

It followed news that Washington had asked Greece to bar Russian supply flights over its territory, triggering an incredulous response from Moscow.

NATO Secretary General Jens Stoltenberg reiterated concerns that increased Russian involvement could deepen the Syrian conflict that has already claimed nearly 250,000 lives and triggered a massive outflow of refugees, tens of thousands of whom are seeking protection in Europe.

"I am concerned by reports that Russia may have deployed military personnel and aircraft to Syria," Stoltenberg said in a public question and answer session on his Facebook page, in response to a question submitted by AFP.

Bulgaria said it had received reports that the Russian planes were carrying arms.

"The cargo was declared as humanitarian aid but the foreign ministry had information that it was not humanitarian aid but some kind of weaponry," said Defence Minister Nikolay Nenchev on local TV station bTV.

He said the information likely came from "our foreign intelligence services".

Foreign ministry spokeswoman Betina Zhoteva told AFP the decision was taken "absolutely independently" without pressure from NATO partners.

"The planes were said to carry humanitarian aid but we had information -- that we had every reason to trust -- that the declared cargo was not the real one," she said.

Greece, also a NATO member, confirmed on Monday it received a request from Washington to prevent two Russian planes flying through its airspace between September 1 and 24.

Speaking to AFP on Tuesday, a Greek government spokesperson said Russia had first requested use of its airspace "25 days ago" but had later decided to use an alternative route.

Asked if Athens would refuse permission for Russian overflights in future, the spokesperson said the situation was "delicate" but that Russia would likely avoid using the route.

- 'On their conscience -

Nonetheless, on Tuesday Moscow demanded answers from Greece and Bulgaria.

"If anyone -- in this case our Greek and Bulgarian partners -- has any doubts, then they of course should explain what the problem is all about," deputy foreign minister Mikhail Bogdanov told Interfax news agency.

"We explain where our planes are flying to, and what their purpose and their cargo is. We've never had any problems before," he added.

Bogdanov reiterated Moscow's official position, stressing that the only Russian military personnel currently in Syria were there to train the local army.

"Our partners -- representatives of the Syrian armed forces -- need some help and guidance," he said.

He denied Russia had any plans to boost its presence in the war-torn country and turn its naval facility at the Syrian port of Tartus into a fully-fledged base.

"We do not have such plans," he said.

Some Russian officials saw Washington's hand in Bulgaria's decision.

"The fact that the Bulgarians are the first to respond (to the request from Washington) -- they will have that on their conscience," said Vladimir Djabarov, vice-president of the Russian foreign affairs council.

He told state press agency TASS that Russia was only supplying "humanitarian cargo" to Syria, saying it was "not profitable to transport weapons by plane".

Djabarov also dismissed the importance of using Greek airspace, saying that the majority of flights to Syria went via the Caucasus and Iran.

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