Satellite imagery revealed that North Korea was still operating its main nuclear site, despite pledges to denuclearize, a monitor said on Wednesday.
Pyongyang is carrying out rapid improvements to its Yongbyon nuclear site and infrastructure works, said the respected 38 North website.
During a historic Singapore summit with US President Donald Trump earlier this month, North Korean leader Kim Jong Un declared a commitment to the denuclearization of the Korean peninsula.
Kim promised to "work toward" the goal, but the Singapore meeting failed to clearly define denuclearization or produce a specific timeline towards dismantling the North's atomic weapons arsenal.
Trump claimed the process would start quickly, saying last week that "it will be a total denuclearization, which is already taking place".
The satellite imagery revealed on Wednesday contradict these pledges.
"Commercial satellite imagery from June 21 indicates that improvements to the infrastructure at... Yongbyon Nuclear Scientific Research Center are continuing at a rapid pace," it said.
It noted "continued operations" at the North's uranium enrichment plant and several new installations at the site -- including an engineering office and a driveway to a building housing a nuclear reactor.
But continued operations at the site "should not be seen as having any relationship with North Korea's pledge to denuclearize", it added.
Nuclear officials could be "expected to proceed with business as usual until specific orders are issued from Pyongyang", it said.
The North last month blew up its aged but only nuclear test site at Punggye-ri -- where it had staged six atomic tests -- in a show of goodwill before the summit.
But Pyongyang has kept its counsel on the denuclearization issue since the meeting, although state media have dialed down propaganda against the US, long dubbed the "imperialist enemy".
North Korea was addressed during talks held between US Secretary of Defense James Mattis and his Chinese counterpart Wei Fenghe in Beijing on Wednesday.
The two officials tackled the thorny issue of how to get North Korea to fulfill its vow to abandon its nuclear program.
Mattis is set to travel to Seoul on Thursday and later Japan as part of an Asian tour.
