ELKO, Nev/MOSCOW (Reuters) - President Donald Trump said Washington will exit the Cold-War era treaty that eliminated a class of nuclear weapons due to Russian violations, triggering a warning of retaliatory measures from Moscow. The Intermediate-Range Nuclear Forces Treaty, negotiated by then-President Ronald Reagan and Soviet leader Mikhail Gorbachev in 1987, required elimination of short-range and intermediate-range nuclear and conventional missiles by both countries.