By Fredrik Dahl VIENNA (Reuters) - The United States and Britain took part for the first time on Monday in an international conference on the humanitarian impact of nuclear weapons and were expected to face pressure to do more to eliminate their atomic arsenals. The three other officially recognized nuclear weapon states -- Russia, France and China -- shunned the Dec. 8-9 meeting. Many of the 157 states taking part are critical of what they see as too slow headway on nuclear disarmament. ...