Turkey says proposed working group to ease U.S. worries over Russian S-400s

ANKARA/ISTANBUL (Reuters) - Turkey has proposed to the United States that they form a working group to determine that Russian S-400 missile defense systems do not pose a threat to U.S. or NATO military equipment, Turkish Foreign Minister Mevlut Cavusoglu said on Wednesday.The United States and Turkey have been at loggerheads over Ankara's decision to purchase the S-400s, which are not compatible with NATO systems, from Russia.Washington has warned that proceeding with the deal could result in U.