The U.K. and U.S. are pressuring Russia to fess up to its alleged role in the near deadly use of a nerve agent on a former spy in southern England, warning that the undeterred use of internationally banned chemical weapons could lead to an increase in their use.U.S. Ambassador to the United Nations Nikki Haley said Russia must come clean about its chemical weapons program and needs to fully cooperate in the U.K.’s investigation into the March 4 attack on Sergei Skripal, a former Russian intelligence officer convicted of spying for the U.K., and his daughter, Yulia.