An aide to Kim said South Korean intelligence had noticed workers and materials moving at the port, where work on the sub appeared to be taking place at an indoor facility.
Secretary of State Mike Pompeo gifted North Korean leader Kim Jong Un with an Elton John CD that includes his hit song "Rocket Man", signed by President Donald Trump, North Korean media reported Friday.
According to a diplomatic source who spoke with Chosunilbo, the "Rocket Man" CD stems from a conversation Trump and Kim had during their denuclearization summit last month.
Now, he'll be able to hear the tune, and maybe return the favour with a mix tape.
Kim's right hand man Kim Yong-chol welcomed Mr Pompeo to North Korea, noting that it was his third visit to the country.
Pompeo arrived at Pyongyang's Sunan Airport on Friday afternoon, where Kim and other officials, including Foreign Minister Ri Yong Ho, Kim Chang Son, a senior official at the State Affairs Commission, and others were at the tarmac, ready to greet him and the US delegation, South Korean news service EDaily reported. Pompeo will stay in Pyongyang for two days, according to The Chosun Ilbo, and discuss North Korea's dismantlement of a specified amount of nuclear weapons with a specified time frame.
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He also attempted to use his influence to get his wife a job, including as the owner of a Chick-fil-A franchise. Facebook friends of Mink's were more than happy to cheer her on: "I am so proud of you!" said one commenter.
U.S. military intelligence noticed similar activity at the port late past year, detecting what appeared to be construction on a new diesel-electric submarine at the Sinpo shipyard, The Diplomat reported in October, citing a USA government source.
Pomepo said yes and added, "I'm looking forward to our time together today".
Speaking on Thursday, Mr Trump told reporters aboard Air Force One on a trip to Montana that he was still upbeat about the future of North Korea and his relationship with its leader.
"On this trip I'm seeking to fill in some details on those commitments and continue the momentum toward implementation of what the two leaders promised each other and the world", Pompeo was quoted as telling reporters during a brief stopover in Japan earlier in the day.
While missile and nuclear tests have halted, one official said, "there's no evidence that they are decreasing stockpiles, or that they have stopped their production".
An underwater test-firing of a submarine ballistic missile shown in an undated photo released by North Korea's Korean Central News Agency on April 24, 2016. We'll see. All I can tell you is this.
News reports over the weekend cited US intelligence sources as questioning the North's commitment to abandoning its nuclear arsenal.