SEOUL, South Korea — Satellite imagery suggests that North Korea may have begun producing fuel rods for its recently restarted nuclear reactor, a United States-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.

The five-megawatt reactor was restarted this year after a six-year hiatus. Its ability to produce plutonium again depends in part on how quickly North Korea can supply it with new fuel rods. North Korea is believed to have only 2,000 fuel rods in its inventory, a quarter of the 8,000 needed for a full load of fuel.

The U.S.-Korea Institute at Johns Hopkins University reported Tuesday on its website, 38 North, that a building in Yongbyon retooled to produce fresh fuel rods for the reactor appeared to be operational. The institute said it had reached that conclusion by analyzing commercial satellite images of the complex.