North Korea, under leader Kim Jong Un, is nearing the capability of striking the continental United States with a nuclear weapon, according to a former U.S. defense secretary. In this undated photo, Kim inspects a factory run by the Korean People's Army, in Pyongyang, on March 18, 2015. Reuters

North Korea is prepared to launch nuclear missiles at any time, a North Korean diplomat told Sky News in an exclusive interview published Friday. Hyun Hak-bong, North Korea's ambassador to Britain, said North Korean officials are closely watching U.S. military exercises in the Pacific amid concerns of a looming invasion.

"We are prepared," Hyun said. "That is why I say if a sparkle of a fire is made on the Korean Peninsula, it will lead to a nuclear war. We don't say empty words. We mean what we mean. It is not the United States that has a monopoly on nuclear weapons strikes."

A Sky News reporter further probed the ambassador, asking, "So can I just be clear? You are telling me that North Korea has the ability now to fire a nuclear missile?"

Hyun replied: "Anytime, anytime. Yes."

North Korea has long threatened the United States with nuclear weapons and the Pentagon has said it takes such threats seriously. It issued a strategy document last year that said North Korea represented “a significant threat to peace and stability on the Korean Peninsula and in Northeast Asia,” while calling it “a growing, direct threat to the United States,” Agence France-Presse reported.

North Korea reportedly has enough weapons-grade plutonium for six nuclear bombs, according to the BBC. Former U.S. Defense Secretary Robert Gates said in 2011 that the Communist state was within five years of being able to attack the continental United States, according to the New York Times. North Korea also has an arsenal of short- and medium-range missiles that poses a threat to South Korea, Japan and other U.S. allies in the Asia-Pacific region.

Meanwhile, the United Nations vowed this week to investigate reports that more than 20,000 North Koreans were forced to travel to Qatar to help build facilities for the 2022 World Cup soccer tournament under inhumane conditions.