File Photo: Russian servicemen drive S-400 missile air defense systems through Red Square in Moscow during the Victory Day parade on May 9, 2018 (Photo: Reuters)
Turkey's purchase of Russian S-400 defence systems is a very serious concern as Washington considers what support to offer over Syria's Idlib, the US special representative for the region James Jeffrey said on Thursday.
Speaking in Istanbul, Jeffrey also said the United States had already offered humanitarian assistance and information sharing with Turkey and was pressing European allies for a significant contribution.
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