North Korea might be trying to extract plutonium to make more nuclear weapons, photos suggest.

SEOUL, South Korea — North Korea may be trying to extract plutonium to make more nuclear weapons at its main atomic complex, recent satellite photos indicated, weeks after leader Kim Jong Un vowed to expand his nuclear arsenal.

The 38 North website, which specializes in North Korea studies, cited the imagery as indicating that a coal-fired steam plant at the North’s Yongbyon nuclear complex is in operation after about a two-year hiatus. Smoke was observed emanating from the plant’s smokestack at various times from late February and early March.

This suggests “preparations for spent fuel reprocessing could be underway to extract plutonium needed for North Korea’s nuclear weapon,” the website said Wednesday. But it added that “this could also mean simply the facility is being prepped to handle radioactive waste.”

Earlier this week, International Atomic Energy Agency Director General Rafael Mariano Grossi said some nuclear facilities in North Korea continued to operate, citing the operation of the steam plant that serves the radiochemical laboratory at Yongbyon. The laboratory is a facility where plutonium is extracted by reprocessing spent fuel rods removed from reactors.

“The DPRK’S nuclear activities remain a cause for serious concern. The continuation of the DPRK’S nuclear program is a clear violation of relevant U.N. Security Council resolutions and is deeply regrettable,” Grossi told the IAEA’S board of governors, according to IAEA’S website.

Plutonium is one of the two key ingredients to build nuclear weapons, along with highly enriched uranium. The Yongbyon complex, north of the capital city of Pyongyang, has facilities to produce both.