Story highlights IHS Jane's says it has identified two sites where Russian troops may arrive

Pentagon says it's "keeping a very close eye on what we see moving in there"

(CNN) Russia may be preparing to station troops at two new sites in Syria as it continues its rapid military buildup in the conflict-ravaged nation, a research firm says.

IHS Jane's said Tuesday that it had spotted two previously unreported sites in satellite imagery of western Syria where steps appear to be being taken to receive Russian forces.

The U.S. government, which has been monitoring with concern the influx of Russian troops and military hardware into Syria, said last week that Russia appeared to be setting up a base near Latakia, a port city on Syria's Mediterranean coast.

Moscow is a staunch ally of Syrian President Bashar al-Assad, whom the United States has repeatedly called on to step down to help bring about an end to the bloody civil war.

U.S. keeping 'very close eye' on Russian moves

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