The United Nations' atomic watchdog has said it has not seen any signs that North Korea has halted its nuclear activities — including those at secret sites — despite its vows to work toward denuclearization at a landmark summit with the U.S. in June.

"The continuation and further development of the DPRK's nuclear programme and related statements by the DPRK are a cause for grave concern," the International Atomic Energy Agency said in a report released late Monday, using the acronym for the North's formal name, the Democratic People's Republic of Korea.

The report by IAEA Director-General Yukiya Amano, which is to be submitted to a board meeting of the body next month, characterized the North's continuing activities at its nuclear facilities as "deeply regrettable."