MOSCOW, Aug 23 (Reuters) - Russia and Turkey are close to signing a new contract to supply Ankara with additional S-400 air defence units in the near future, the Interfax news agency cited the head of Russia"s Rosoboronexport arms exporter as saying on Monday. Turkey"s initial purchase of S-400s from Russia strained ties with the United States and its NATO allies. Washington sanctioned Ankara and has threatened to impose new sanctions if it buys more major arms systems from Moscow. read more