IAEA: North Korea is increasing its nuclear weapons capabilities

North Korea has made "very serious" advances in nuclear weapons production and may be opening a new uranium enrichment facility, the United Nations (UN) Atomic Energy Agency (IAEA) said.

IAEA chief Rafael Grossi confirmed in Seoul a sharp increase in the productivity of the five-megawatt reactor, the reprocessing unit and other facilities at the Yongbyon nuclear complex, Reuters reports.

North Korea's nuclear program is estimated to have several dozen warheads, Grossi told reporters, speaking of growing activity at parts of one of the facilities.

"All of this activity points to a serious increase in North Korea's nuclear capabilities," he added.

Grossi told a meeting of IAEA officials earlier this month that the agency was closely monitoring the new building of the Yongbyon complex, which has some similarities to the uranium enrichment plant in Kangson, near Pyongyang.

Grossi said the agency had not seen evidence of the use of Russian technology in North Korea's nuclear program.

The cooperation agreement signed by Moscow and Pyongyang last year appears to apply only to civilian nuclear projects, although there are still no concrete conclusions, he added.

"A turn or advance towards nuclear weapons will never enhance the security of a state, but can rather provoke proliferation," Grossi added.

The IAEA chief added that he called on South Korea to work more closely with the agency to avoid and repel proliferation risks, referring to the country's program to build nuclear-powered submarines.