SEOUL — Satellite imagery suggests that North Korea may have begun producing fuel rods for its recently restarted nuclear reactor, a US-based research institute said in a report published Tuesday.

The signs of new activity at North Korea's main nuclear complex in Yongbyon, north of Pyongyang, follow the country's repeated assertions that it is strengthening its capabilities to produce nuclear arms.

North Korea, which has conducted three nuclear tests since 2006, the most recent in February, has used spent fuel rods from the reactor as a source for plutonium, a key component for nuclear weapons.