ANKARA (Reuters) - Turkey condemned as a “grave mistake” U.S. sanctions over its purchase of Russian S-400 missile defences, and said on Monday it will retaliate as necessary after it repeatedly called on Washington to solve the issue through dialogue. Earlier on Monday the United States acted on its long-threatened move and imposed sanctions on the top Turkish defense development body, its president and three employees. The Turkish Foreign Ministry said sanctions were “inexplicable” after Washington repeatedly rejected Ankara’s offer to form a joint working group. It called on its NATO ally to revise the “unjust” decision that will harm bilateral ties.