North Korea will probably claim with credibility within four years that it can hit the U.S. with a nuclear weapon, a potential time bomb for Donald Trump's re-election prospects, according to Christopher Hill, a former senior U.S. diplomat who led talks with the reclusive regime.

The chance of multicountry negotiations resuming soon with North Korea is "pretty much nil," Hill said Saturday in an interview in Singapore. The former assistant secretary of state for East Asian and Pacific affairs, who was ambassador to South Korea from 2004-2005, said Kim Jong Un was likely to keep improving his arsenal and the options for countering him were limited.

"Contrary to what many people have said, they are not testing the new administration, they are testing weapons of war," Hill said. "And sooner or later, but certainly within the next four years, within the first mandate of Trump, they will announce with credibility that they have a deliverable nuclear weapon."