NATO Secretary General Jens Stoltenberg expressed his concern on Tuesday after reports emerged that Russia could be transporting military aid to Syria.

"Yes, I am concerned by reports that Russia may have deployed military personnel and aircraft to Syria," Stoltenberg said in a question and answer session on Facebook. "Any military support to the Assad regime could further escalate the conflict."

Stoltenberg's comments came a few hours after Bulgaria closed its airspace to Russian military planes bound for Syria, saying it had reason to believe the "declared cargo was not the real one."

The Kremlin struck back, asking Bulgaria to explain why it had banned its aid flights to Sanaa and that it had never hidden the fact that it supplies military equipment to Syria to "fight terrorism." A high-ranking Moscow official warned of reprisals.

Russia and Syria are close allies, as seen in talks between Syrian Foreign Minister Walid al-Moallem (left) and his Russian counterpart Sergey Lavrov

"The issue is not even about the retaliatory measures that Russia will take against Bulgaria in one way or another… The issue is about the fact that this undermines the future of our relations," said Leonid Kalashnikov, the deputy head of the Duma's international affairs committee, according to the Russian Tass news agency.

Strained relations with Moscow

Moscow is a close ally of President Bashar al-Assad's government in Syria. Russia has not yet sent troops to the conflict zone, but has been ramping up its role as a mediator in the country's civil war.

But NATO countries including the US, which leads the military coalition fighting the "Islamic State" in Syria and the Middle East, has warned that Russia's intervention may deepen the crisis, in which more than 200,000 people have died.

The latest controversy adds more tension to the already strained relations between Western countries and Russia, which has been the target of economic sanctions for its alleged role in supporting eastern Ukraine's rebels and the annexation of Crimea last year.

NATO has responded to Moscow's aggression by beefing up strategic positions and organizing military drills in its new Eastern European members.

mg/cmk (AFP, Reuters, dpa)