North Korea is continuing to produce fissile material for its nuclear weapons at its main nuclear research site in Yongbyon and new construction is underway to readjust the facilities, the International Atomic Energy Agency said in an annual report.

"There were indications consistent with the reactor's operation, including steam discharges and the outflow of cooling water," the IAEA said in a recent report, titled "Application of Safeguards in the Democratic People's Republic of Korea."

"Based on past operational cycles, the current cycle could be expected to continue until late 2017," the report said, referring to a 5-megawatt experimental reactor at the research site.

"There were indications consistent with the use of the reported centrifuge enrichment facility located within the plant," the report also said, suggesting North Korea's continuing uranium enrichment. "Construction work was undertaken on a building that adjoins the reported centrifuge enrichment facility."

An increase in activity has also been monitored at the construction site for a light water reactor on the compound, the report also said, suggesting progress in North Korea's ongoing development of indigenous light water reactor nuclear power station technology.

"There were indications in the LWR construction yard of an increase in activities consistent with the fabrication of certain reactor components ... Work to connect what appears to be the LWR's electrical switchyard with the electrical distribution network was completed," it noted.

There were also "new construction and refurbishment activities" on the site which are consistent with the North's announcement in 2015 that all the nuclear facilities, including the uranium enrichment plan and 5MW reactor, were "rearranged, changed and readjusted" and had started normal operations, according to the IAEA report.

Meanwhile, the report said the research site's radiochemical laboratory, which involved processing plutonium in the past, appears to have not been in operation since last year.

"The continuation and further development of the DPRK's nuclear program and related statements are a cause for grave concerns," the IAEA said.

"The DPRK's nuclear activities, including those in relation to the Yongbyon Experimental Nuclear Power Plant reactor, the use of the building which houses the reported enrichment facility, and the construction at the LWR are deeply regrettable," according to the organization.

It warned such actions are "clear violations" of relevant United Nations Security Council resolutions, calling on the country to comply fully with the resolutions and cooperate with the IAEA in resolving all outstanding issues.

The IAEA currently has no access to the North Korea nuclear site and monitors the country mainly through satellite imagery. (Yonhap)