Korean leader Kim Jong Un vowed last January to enlarge nuclear arsenal and disclose an array of high-tech weapons systems targeting the United States, recent satellite photos indicated that North Korea may be trying to extract plutonium to make more nuclear weapons at its main atomic complex.

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. DPRK refers to North Korea’s official name, the Democratic People’s Republic of Korea.

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 ingredients. It’s not clear exactly how much weapons-grade plutonium or highly enriched uranium has been produced at Yonbyong and where North Korea stores it.

Outside estimates on North Korea’s nuclear arsenal vary. In 2018, a South Korean official told Parliament the North may have 20 bombs to as many as 60.

U.S.-led diplomacy aimed at persuading North Korea to abandon its nuclear program in return for economic and political benefits has been deadlocked since a summit between President Donald Trump and Kim collapsed in early 2019. Trump rejected Kim’s calls for extensive sanctions relief in return for dismantling the Yongbyon complex in what was seen as a limited denuclearization step because North Korea had already built nuclear weapons and is believed to be running other covert bomb-making facilities.