Turkey announced September 12, 2017, that it has signed a deal with Russia to purchase the Russian S-400 surface-to- air missile defense system. President Recep Tayyip Erdogan told the press that a deposit has already been paid. Though the prospective purchase was made public several months ago, this announcement is the first time Erdogan has confirmed a money transfer for the missile system. The deal is estimated to be worth $2.5 billion. The United States has already voiced concerns about the purchase of the Russian system, which will not be interoperable with other NATO systems.