The alarms are sounding in Washington.
According to a Syrian military official, Russian military experts have become more present in Syria over the last year.
"Russian experts are always present but in the last year they have been present to a greater degree," the official said.
Intelligence reports triggered US Secretary of State John Kerry to call Russian foreign minister Sergey Lavrov on Friday, and warn about the consequences of Russia expanding its military role in Syria, The New York Times reports.
Following the call, a spokeswoman for Lavrov said that "the Kremlin had long provided military assistance to the Syrian government in its fight against extremists and expressed surprise at Kerry’s warning."
However, as The New York Times notes:
Russia appeared to be making preparations to deploy advisers and military personnel to an airfield south of Latakia and might also bring in aircraft and fly airstrikes from there.
Those preparations included the delivery of prefabricated housing for as many as 1,000 personnel and a portable air traffic control station to the airfield.
Over the weekend, two giant Russian Condor transport planes ferried more supplies and equipment from an air base in southern Russia across Iran and Iraq to Latakia, according to an American official who spoke on the condition of anonymity because the official was discussing intelligence reports.
"They’re clearly establishing some sort of forward operating base," an unidentified US official reportedly said.
Syrian President Bashar Assad's weakening regime isn't good for Russian President Vladimir Putin.
"If the West succeeds in turning the tide of the war while Assad is vulnerable, the political outcomes in Syria are more likely to be dictated by the US,"geopolitical expert Ian Bremmer, the president of Eurasia Group, told Business Insider.
"Which means Putin needs to bolster Assad now," Bremmer added.
Since Syria's civil war began in 2011, regime barrel bombs have killed more than 12,000 people, dwarfing the combined number killed by ISIS and Al Qaeda.
Michael B. Kelley contributed to this report.
SEE ALSO: BREMMER: Putin is serious about entering Syria — and not to go after ISIS
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