At left, Russian Foreign Minister Sergey Lavrov; at right, U.S. Secretary of State John Kerry. (AP)

Moscow is continuing to send military cargo flights to Syria despite Secretary of State John Kerry calling 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 that the U.S. military is tracking multiple flights of Russia's largest military cargo plane, the Antonov An-124 Ruslan -- better known by its NATO codename, "Condor." 

This includes 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. 

This is the first time in months the U.S. military has seen flights of the large Russian transport planes, one official said. The official called this development "very troubling." 

In a statement released Saturday after Kerry's call with Foreign Minister 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 Syria's Bashar 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, 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 last Thursday. 

"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. Over the weekend, the AP also reported that Greece has asked Russia to stop overflights into Syria, at the request of the U.S. government. 

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.

Lucas Tomlinson is the Pentagon and State Department producer for Fox News Channel. You can follow him on Twitter: @LucasFoxNews