MOSCOW (Reuters) - Russia has sold its advanced S-400 air defense missile system to Turkey for over $2 billion, Sergei Chemezov, the head of state conglomerate Rostec, said on Thursday, the TASS news agency reported.

The deal with Turkey, a NATO member, has caused some disquiet in the United States.

Turkey originally awarded a $3.4 billion contract for a defense system to China in 2013, but canceled that two years later, saying it would concentrate on developing a system domestically.