MOSCOW, Oct 17 (Reuters) - The Kremlin on Thursday questioned the tone of a letter sent by U.S. President Donald Trump to his Turkish counterpart Tayyip Erdogan, saying it was highly unusual for correspondence between heads of state. The White House on Wednesday released the Oct. 9 letter, in which Trump urged Erdogan to halt Turkey's cross-border offensive into northeast Syria, saying "Don't be a tough guy" and "Don't be a fool!"