Khamenei condemns Republican letter on Iran, bemoans U.S. "tricks"

* Iran's top leader says Washington known for "backstabbing" * Republican senators suggest no deal with Iran may stand * White House: Senators' move was "reckless" interference By Sam Wilkin DUBAI, March 12 (Reuters) - Iran's Supreme Leader hit out on Thursday at a letter by U.S. Republican senators threatening to undo any nuclear deal between Washington and Tehran, saying he was worried because the United States was known for "backstabbing", Mehr news agency reported. Ayatollah Ali Khamenei, th