"This is an extraordinary issue. We call on Turkey at all levels and at every opportunity not to keep its S-400 missile system and to refrain from purchasing additional Russian military equipment," Deputy Secretary of State Wendy Sherman said at a news conference on Friday.

Sherman added that the United States "clearly clarifies to Turkey the implications it will face in the event of progress in this direction."

This comes following the announcement by Turkish President Recep Tayyip Erdogan last Thursday, at the end of his meeting with his Russian counterpart Vladimir Putin at the Sochi summit, that Turkey is considering the possibility of purchasing new weapons and military equipment from Russia.