The development comes against the backdrop of a Syrian army onslaught on the rebel-held eastern part of the city of Aleppo in northern Syria, which has been backed by Russian warplanes. Since the collapse of a US-Russia-brokered cease-fire last month, ties between Moscow and Washington have grown even more strained.
The two powers support opposite sides in the war - Moscow has been a staunch Assad ally and the United States backs Syrian rebels trying to oust him.
Lawmakers in the Kremlin-controlled State Duma voted unanimously to approve the deal, which allows Russia to keep its forces at the Hemeimeem air base in Syria's coastal province of Latakia, Assad's Alawite heartland, for as long as it wants. The deal was signed in August 2015 in Damascus, a month before the Russian air campaign began.
Yesterday, the Russian military warned the US against striking the Syrian army, stressing that Russian air defense weapons in Syria stand ready to fend off any attack.
Russian Defense Ministry spokesman Maj Gen Igor Konashenkov said any US strikes on areas controlled by Assad's government could jeopardise the lives of Russian servicemen, and noted that the range of Russian S-300 and S-400 air defense missile systems would "surprise" any country operating its aircraft over Syria.
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