VIENNA (Reuters) -Western powers are preparing to push the U.N. nuclear watchdog's board at its next quarterly meeting to declare Iran in breach of its non-proliferation obligations for the first time in almost 20 years, a move bound to enrage Tehran, diplomats said. The step is likely to further complicate talks between the United States and Iran aimed at imposing fresh restrictions on Iran's rapidly advancing nuclear programme. Washington and its European allies Britain, France and Germany, known as the E3, proposed past resolutions adopted by the International Atomic Energy Agency's 35-nation Board of Governors calling on Iran to quickly take steps such as explain uranium traces the IAEA found at undeclared sites.