Russia's Foreign Ministry disclosed early Wednesday that Russian military experts are assisting forces loyal to Syrian President Bashar al-Assad in that country's long-running civil war.

The statement by Foreign Ministry spokeswoman Maria Zakharova to Reuters marks the first confirmation from Moscow that members of the Russian military are on the ground in Syria after weeks of reports that Russia had stepped up its support for the embattled Assad regime.

Zakharova said the advisers were assisting with Russian arms deliveries to Syrian government forces, which Moscow says are aimed at fighting Islamist militants. The spokeswoman did not give a precise number of Russian military personnel in Syria.

The disclosure comes as U.S. officials have expressed increasing concern about the number of Russian military cargo flights to Syria. Secretary of State John Kerry called his Russian counterpart Sergey Lavrov over the weekend to voice "concerns" about the "imminent" buildup of Russian military forces there.

Multiple U.S. officials who have reviewed the latest intelligence in Syria told Fox News Tuesday that the U.S. military was tracking multiple flights of Russia's largest military cargo plane, the Antonov An-124 Ruslan -- better known by its NATO codename, "Condor."

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Notably, the "Condor" made one flight Tuesday morning into Latakia, a Syrian port city along the Mediterranean Sea controlled by the Assad regime and home to Russian military forces.

The flights marked the first time in months the U.S. military had seen flights of the large Russian transport planes, one official said, calling this development "very troubling."

Last week, the Times of London reported that video shot near Latakia and shown on Syrian state TV showed Syrian government troops supported by a Russian armored vehicle. Russian-speaking voices were also heard in the background of the footage.

On Wednesday, the Russian Foreign Ministry also accused the U.S. of "international boorishness" over Washington's requests to Greece and Bulgaria to deny the cargo flights the use of those countries' airspace. Bulgaria's Foreign Ministry has refused to allow Russian military aircraft to fly over its territory until Sept. 24 at the earliest. On Monday, Greek Foreign Ministry spokesman Constantinos Koutras said Athens was examining a similar request from the U.S.

In a statement released Saturday after Kerry's call with Lavrov, the State Department warned that Russia's continued actions could "further escalate the conflict" and "risk confrontation with the anti-ISIL Coalition operating in Syria."

The statement appeared to mark a change in policy from last week when the Pentagon and State Department initially welcomed Russia's increased involvement in Syria.

Vladimir Putin's government is aligned with Assad, whom the Obama administration wants out of power. But the administration also is boosting local forces fighting the Islamic State in Iraq and Syria (ISIS), and initially cast the stepped-up Russian involvement as contributing toward that anti-ISIS effort.

"We would welcome anyone who wants to help in the coalition against ISIL," Pentagon Press Secretary Peter Cook said last Thursday, using another acronym for the terror group.

"There's a 37-some-odd-country coalition that's taking the fight to ISIL. We would welcome Russia to be more involved in that effort," State Department spokesman Mark Toner also said.

The statement following the Kerry-Lavrov call, though, indicates the Obama administration now worries Russia's involvement could make the situation worse.

Lavrov later remarked that Russia has been continuously supplying military equipment to Syria. He said Moscow "has never concealed that it delivers military equipment to official Syrian authorities with the aim of combating terrorism," according to Reuters.

Putin is expected to attend the U.N. General Assembly for the first time in 10 years later this month in New York.

Fox News' Lucas Tomlinson and the Associated Press contributed to this report.