Russian forces have joined military operations supporting government troops in Syria, Reuters reports, citing three unnamed Lebanese sources.

The claim comes after the Russian Foreign Ministry confirmed the presence of military experts in Syria, but said they were there to oversee arms deliveries in the fight against terrorism.

NATO secretary general Jens Stoltenberg called Russia’s presence a cause for concern. Washington echoed his worry.

“We would welcome constructive Russian contributions to the counter-[ISIS] effort, but we’ve been clear that it would be unconscionable for any party, including the Russians, to provide any support to the Assad regime,” White House spokesman Eric Schultz said, referring to the Islamic State of Iraq and Greater Syria, according to Reuters.

The scale of Russia’s military presence is not yet confirmed. But one of the Lebanese sources told Reuters the number of Russian troops is still small. The news agency also cited U.S. officials who said that Russia had sent two tank landing ships and a small number of marines to Syria.

To block increased military buildup in Syria, the U.S. has asked Bulgaria and Greece to close their airspaces to Russia transport planes. Russia called this request “international boorishness.”

Rising tension between the two nations comes before Russian President Vladimir Putin is expected to visit the U.S. for the U.N. General Assembly later this month.

[Reuters]

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Write to Mark Rivett-Carnac at mark.rivett-carnac@timeasia.com.