The U.N. nuclear watchdog said it did not find any indication that North Korea had stopped its nuclear activities, adding to doubts about the country's willingness to abandon its arsenal.
"The continuation and further development of the DPRK's nuclear program and related statements by the DPRK are a cause for grave concern," the International Atomic Energy Agency (IAEA) said in a report published late Monday.
The report, which refers to the country's official name Democratic People's Republic of Korea, is to be submitted to the IAEA's board meeting next month.
U.S. President Donald Trump said in an interview with Reuters on Monday he believed North Korea had taken specific steps toward denuclearization and that he would "most likely" meet again with North Korean leader Kim Jong Un.
Trump held a landmark summit with Kim on June 12, at which the North Korean leader agreed in broad terms to work toward denuclearization of the Korean peninsula.
However, the country has given no indication it is willing to give up its weapons unilaterally as the Trump administration has demanded.
"As the agency remains unable to carry out verification activities in the DPRK, its knowledge of the DPRK's nuclear program is limited and, as further nuclear activities take place in the country, this knowledge is declining," the IAEA said.
Between late April and early May there were indications of the operation of the steam plant that serves a radiochemical laboratory, the report said. However, the duration of the steam plant's operation was not sufficient to have supported the reprocessing of a complete core from the experimental nuclear power plant reactor.
Dismantling Pyongyang's nuclear weapons program and verifying it would be a large and complex task. The IAEA has said it is best placed to verify a deal.