The possible signs of fresh reprocessing activity last week come after a February summit between US President Donald Trump and North Korean leader Kim Jong Un ended abruptly without agreement on Pyongyang's nuclear programme.
A view of rail cars near the isotope/tritium production facility, as seen on April 12.
A view of what researchers of Beyond Parallel, a CSIS project, describe as a probably 20-foot shipping container near the uranium enrichment plant at the Yongbyon Nuclear Research Center in North Pyongan Province, North Korea, in this commercial satellite image taken April 12, 2019 and released April 16.
It said their movement could indicate the transfer of radioactive material.
Breaking his silence on the summit in a speech to DPRK's Supreme People's Assembly on Friday, Kim said it was "essential for the U.S.to quit its current calculation method and approach us with a new one".
Jenny Town, a North Korea expert at the Stimson Center think tank, said that if reprocessing was taking place, it would be a significant given U.S.
"I agree with Kim Jong Un of North Korea that our personal relationship remains very good, perhaps the term excellent would be even more accurate, and that a third Summit would be good in that we fully understand where we each stand". -North Korean talks in the past year and the failure to reach an agreement on the future of Yongbyon in Hanoi.
Trump and Kim left the Vietnamese capital without a deal in part due to disagreements over trading sanctions relief for North Korea scaling back its nuclear program. He rejected partial denuclearization steps offered by Kim, which included an offer to dismantle Yongbyon.
North Korea suspended nuclear and missile testing during the diplomatic process in 2018 but the International Atomic Energy Agency has said there were indications that Yongbyon has been in use as recently as the end of February.
He said his country would "wait for a bold decision from the USA with patience till the end of this year", raising the potential for the unresolved nuclear issue to become a liability for Trump during his 2020 re-election bid.
Last month, a senior North Korean official warned that Kim might rethink the test freeze unless Washington made concessions.
Secretary of State Mike Pompeo is expressing confidence President Trump will be successful in getting North Korea to denuclearize.
Russian Federation has previously echoed North Korea's call for phased sanctions relief in exchange for denuclearization, while Trump has reportedly pushed for a big deal that gets that gets to the heart of North Korea's nuclear program and goes beyond Pyongyang dismantling one key facility, Yongbyon.
"It would underscore that it is an active facility that does increase North Korea's fissile material stocks to increase its arsenal".
Kim Yo Jong, who is considered to be the most powerful woman in the country, was not listed as an alternate member of North Korea's ruling Workers' Party of Korea's (WPK) politburo - the party's top decision-making body - and did not appear at any high-profile events during an important party gathering last week, NK News reported, citing state media.