By Michelle Nichols UNITED NATIONS (Reuters) – The United States will again push for more U.N. sanctions on North Korea if it carries out a seventh nuclear test, despite opposition by China and Russia, U.S. Ambassador to the United Nations Linda Thomas-Greenfield said on Tuesday. China and Russia last week vetoed a U.S.-led push to impose more U.N. sanctions on North Korea over its renewed ballistic missile launches, publicly splitting the U.N. Security Council for the first time since it started punishing Pyongyang in 2006. Thomas-Greenfield has warned that North Korea is “actively preparing …