Russian President Vladimir Putin meets with Syrian leader Bashar al-Assad at the Kremlin in December 2006. (Mikhail Klimentyev/AP)

Russia acknowledged Wednesday that it has military advisers in Syria and left open options to expand its weapons training and assistance amid increasing friction with the West over Moscow’s backing for Syrian President Bashar al-Assad.

Although Russia has provided military support to Assad for years — and the presence of advisers does not signal a policy shift — the extent of Moscow’s aid has taken on added resonance as unprecedented numbers of Syrian refugees and others from the region stream into Europe.

This week, the United States urged Greece and Bulgaria to reject overflight requests for Russian aircraft, fearing that Moscow seeks to increase its arms shipments to Assad’s forces. Iran, however, has agreed to open its airspace to Russian planes carrying “humanitarian cargo” bound for Syria, according to Russia’s TASS news agency.

Russia’s Foreign Ministry called Washington’s intervention “international boorishness” in a statement Wednesday, saying Moscow only seeks to help the Syrian government fight Islamist militants.

On Wednesday, Secretary of State John F. Kerry telephoned Russian Foreign Minister Sergei Lavrov for the second time in five days, telling his counterpart that a Russian buildup in Syria could escalate the conflict there.

“Russia has never made a secret of its military-technological cooperation with Syria. Russian military specialists help Syrians master Russian hardware,” Russian Foreign Ministry spokeswoman Maria Zakharova told journalists in Moscow on Wednesday.

She added that Russia also would consider “extra measures to intensify counterterrorism efforts in Syria” if needed but did not describe any possible plans.

Zakharova also gave no details on the number of Russian military personnel in Syria, but she noted that Moscow is “providing assistance to Syrians in training them to operate Russian military” equipment.

Two U.S. officials quoted by Reuters said that tank-landing ships, additional aircraft and a contingent of naval infantry had recently been deployed to the country.

The officials seem to confirm recent reports on social media that a brigade based in Sevastopol, Crimea, was on its way to Syria.

The Syrian port of Tartus has offered Russian warships a strategic base in the Mediterranean, but it remains unclear how many Russian military personnel are at the site. There also have been reports of Russian forces building an airfield in Latakia province, which is in western Syria, on the Mediterranean. On Tuesday, CBS News reported that three Russian transport aircraft had landed at the airfield in recent days.

Nikolai Fedoryak, first deputy head of the Federation Council committee on defense and security, denied that Russian troops are involved in direct military action in Syria, the Interfax news agency reported.

Russia’s involvement in Syria is part of a longtime Russian role in the region, said John E. Herbst, director of the Atlantic Council’s Dinu Patriciu Eurasia Center.

“There has always been a Russian presence in the Middle East,” Herbst said. “It’s not surprising that they are reasserting themselves in Syria.”

Russia’s pivot to Syria could be due in part to recent setbacks in Ukraine, Herbst added, noting that the buildup in war-torn Middle Eastern country could be used as a diversion of sorts.

Sen. Jack Reed (D-R.I.), the ranking minority party member on the Senate Armed Services Committee, echoed Kerry’s remarks in a meeting Wednesday with reporters but was cautious on how to proceed.

“This situation [in Syria] is chaotic,” Reed said. “Kerry sent a message to Lavrov, and that message has to be delivered, and the next question is: How do they respond?”

A Pentagon spokeswoman, Cmdr. Elissa Smith, said U.S. officials were aware of reports of Russian military personnel and aircraft in Syria and were “monitoring the situation closely.”

Washington has supported Syrian rebel forces opposed to Assad in addition to carrying out airstrikes against Islamic State targets.

On Tuesday, Pentagon press secretary Peter Cook said U.S. officials plan to move forward with a “train and equip program” for so-called moderate rebels despite setbacks that included one U.S.-backed rebel unit’s being overrun by militant fighters.

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