The repatriation of the remains of American service members from the Korean War, which ended 65 years ago, was one of the four points listed in the joint statement President Donald Trump and North Korea's leader Kim Jong Un signed on June 12 during their historic meeting in Singapore.
United States intelligence agencies have found that North Korea is building new missiles, based on satellite photographs taken in recent weeks and other new evidence, The Washington Post reported Monday.
The pictures show that at least one and possibly two liquid-fueled ICBMs are being worked on.
U.S. Secretary of State Mike Pompeo told the U.S. Senate Foreign Relations Committee last week that North Korea was continuing to produce fuel for nuclear bombs despite its pledge to denuclearize.
"Nothing's changed. Our objective remains the final, fully verified denuclearization of North Korea as agreed to by Chairman Kim Jong Un". However, he has given few details of how he plans on doing that. He declined to say whether Pyongyang is constructing new missiles.
Kim Do-gyun, the South's chief negotiator who is in charge of North Korea policy at the Ministry of National Defense, told reporters before leaving for the DMZ that he would make efforts to craft "substantive" measures to ease tensions and build trust.
The remains were handed over last week in the coastal city of Wonsan, North Korea, before they were flown back to an air base in South Korea.
A cut-off of oil and fuel would require enforcement primarily by China, which supplies most of North Korea's energy needs, but also by Russian Federation, which delivers some oil to Pyongyang. Some experts say South Korea can't agree on any drastic measures to reduce animosity unless the North takes serious nuclear disarmament steps. Indonesia guarantees the security of all countries' representatives and contingents, including those of North Korea.
Korean War: North Korea hands over United States soldier remains
A source speaking to Fox News, identified as an American official, gave a similar report of the status of the remains transfer. North Korea has held out the return of remains as a symbol of its goodwill and intention to improve ties with Washington .
She also noted that large "brightly coloured containers" also showed up in satellite imagery, saying that "containers similar to these have appeared during previous ICBM inspections by Mr Kim".
The football-field-size building surrounded by a high wall and high-rise residential towers was externally completed by 2003, according to historical satellite photos, and U.S. intelligence agencies believe it has been operational for at least 10 years.
The findings are the latest to show ongoing activity inside North Korea's nuclear and missile facilities at a time when the country's leaders are engaged in arms talks with the United States.
Sohae has been the main site for North Korea's satellite launches since 2012.
It is possible, the official said, that any new missiles the North is building may be for further testing of such vehicles and of more accurate guidance systems.
Between 1990 and 2005 229 sets of remains from the North were repatriated, but those operations were suspended when ties worsened over Pyongyang's banned nuclear weapons programme.