Sept. 7, 2015: Russian President Vladimir Putin, center, listens to a question during a meeting with his supporters in Moscow, Russia. (AP)
U.S. officials are expressing growing concern about Russia's military build-up in Assad-controlled Syria, calling it "unprecedented" -- with one telling Fox News it compares in scope to Vladimir Putin's incursion into Crimea.
18:39 Syria crisis: 'We're working very hard' for peace, says U.S
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00:18 Russia and Syria pause in bombing of rebel-held sections of Aleppo