WASHINGTON (Reuters) - U.S. Defense Secretary Jim Mattis said on Thursday that he still had confidence in diplomatic efforts on North Korea and the United States would be  unrelenting  in working through the United Nations.  I am not willing to say that diplomacy has not worked. We will continue to work diplomatically, we will continue to work through the United Nations, the United Nations Security Council and we will be unrelenting in that,  Mattis said before the start of a meeting with Libyan Prime Minister Fayez al-Seraj at the Pentagon.  At the same time, our diplomats will speak from a position of strength because we do have military options,  he added. 