In an annual report posted on its website, the International Atomic Energy Agency says it has seen releases of steam and water from a reactor at the Yongbyon nuclear complex.
The latest update on the plant also quoted experts that said the North could be working on making plutonium for atomic bombs.
The UN watchdog said it was continuing to monitor developments at the site via satellite imagery.
The reactor site has technically been out of operation for years, as North Korea destroyed its cooling tower in 2008, amid a trust-building phase with South Korea, China, the U.S. and other neighboring countries.
In April 2013, the North said it would revive its old five-megawatt reactor at the Yongbyon nuclear complex.
Experts said then that it would take about half a year to revive the plant, but only if it hadn't fallen completely into disrepair.
In January, the U.S. Director of National Intelligence said the North had enlarged the uranium enrichment facility, and restarted the reactor as well.
Calling the moves "a significant step in the wrong direction," and a contrary move to the North's international commitments, the U.S. State Department said on Thursday that the activities signal Pyongyang's clear lack of commitment to denuclearization.
Shin Se-min, Arirang News.