Speaking at a security conference in Singapore ahead of a planned summit between President Donald Trump and North Korean leader Kim Jong-Un, Mattis said it is vital that the worldwide community keeps UN Security Council sanctions in place for now. According to South Korea's Yonhap news agency, all three of North Korea's top military leaders have been replaced, the Guardian reports.
U.S. Defense Secretary Jim Mattis said on Sunday North Korea will receive relief only after it takes clear and irreversible steps to end its nuclear program, adding it would be a bumpy road to a summit between U.S. and North Korean leaders.
"The optics are horrendous given that Kim Jong-un is trying to posture as a good guy", Sue Mi Terry, the Korea chair at the Centre for Strategic and International Studies in Washington told the New York Times.
North Korea's nuclear weapons program has been a source of major security tensions that persisted despite a series of United Nations and US sanctions and it has also demonstrated advances in ballistic missile technology that experts believe now threatens the USA mainland.
Reports said the reshuffle could be aimed at eliminating resistance to the peace overtures.
Tensions on the Korean Peninsula have significantly de-escalated in 2018 with North Korean leader Kim Jong-un having twice met with South Korean President Moon Jae-in.
No date has been given for the summit, which would be the first public meeting between Kim and another head of state in Pyongyang.
Mr Trump revived the Singapore summit on Friday after cancelling it a week earlier.
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But North Korea's leadership is believed to regard nuclear weapons as crucial to its survival and has rejected unilaterally disarming.
Defence minister Pak Yong Sik has been suceeded by former first vice minster No Kwang Chol.
In 2016, the Director of National Intelligence, James Clapper, released a report that placed gene editing technology on a par with nuclear weapons as a threat to U.S. national security.
To underscore the challenges in following North Korean affairs, the Yonhap agency reported erroneously in February 2016 that Ri Yong Gil had been executed as part of high-level purges.
All of the newly promoted officials are younger than their predecessors, including 63-year-old Ri Yong Gil, who is 21 years younger than Ri Myong Su. "North Korea will receive relief only when it demonstrates verifiable and irreversible steps to denuclearization", Mattis added.
Should real progress be made, South Korean President Moon Jae-in's plan involves the development of three economic belts that would link his country's industrial heartland with the North and then with China and Russian Federation.