A senior United States official, who declined to be identified, quoted a report by South Korea's Yonhap news agency on Sunday which said all three of the North's top military officials were believed to have been replaced.
"North Korea will receive relief only when it demonstrates verifiable and irreversible steps to denuclearization", Mattis said during public remarks at a meeting with the South Korean and Japanese defense ministers.
North Korea in 1994 and 2005 signed agreements to end its nuclear weapons program in return for diplomatic and economic rewards, but it went on to conduct the first of its six nuclear tests in 2006, bringing about a series of United Nations sanctions. North Korea's leadership is believed to regard nuclear weapons as crucial to its survival, while Kim has said he plans to focus on economic development.
The officials who reportedly were dropped are from some of the highest reaches of the North's military structure, including Ri Myong Su, the chief of general staff for the Korean People's Army.
North Korea's top three military officials have been removed from their posts, a senior US official said on Sunday, a move analysts said could support efforts by the North's young leader to jump-start economic development and engage with the world.
"I am going to visit the DPRK and meet HE Kim Jong-un", Assad was quoted saying in the report, using the initials for North Korea's official name and referring to Kim using letters that stand for "his excellency".
Citing an unnamed intelligence official, Yonhap said No Kwang Chol, first vice minister of the Ministry of People's Armed Forces, had replaced Pak Yong Sik as defense chief, while Ri Myong Su was replaced by his deputy, Ri Yong Gil.
All three appear to have been replaced by younger Kim loyalists, part of an ongoing transformation of the country's political and military establishment since the young leader took power in 2011.
"The three ministers welcomed the results of the two recent Inter-Korean Summits" in April and May, the statement said, adding they "noted the positive changes that have been bought about are setting favorable conditions for the U.S".
Abe said at a joint press conference with Trump on April 18 in Palm Beach, Florida, that they "completely" agreed to keep maximum pressure on Pyongyang, more than one month after the U.S. President chose to hold a summit with North Korean leader Kim Jong Un.
The president made the announcement that the summit was back on after meeting with North Korean official Kim Yong Chol in the Oval Office. "They know to sit there and not get too drunk at the parties. they know how to behave themselves".