TOKYO — North Korea appears to have restarted its main nuclear reactor at Yongbyon in July, a "deeply troubling" sign that the country may be on track to expand its nuclear program, according to a new report by the United Nations' atomic agency.

Since early December 2018, there have been no indications that the main plutonium-producing reactor was in operation. But satellite images taken this year show signs consistent with the operation of the reactor, including the discharge of cooling water, according to an annual report submitted to the International Atomic Energy Agency’s board of governors on Friday.

The finding poses another challenge to the Biden administration’s goal of denuclearizing North Korea. The apparent restarting of the reactor is notable given that the United States’ negotiations with Pyongyang over its nuclear program have stalled since 2019, when North Korean leader Kim Jong Un offered to dismantle the sprawling Yongbyon complex — but not other nuclear and missile sites — in exchange for extensive sanctions relief.

Although Yongbyon is not the only site where North Korea has produced highly enriched uranium, its role at the heart of Pyongyang’s nuclear ambitions made the facility a bargaining chip in negotiations. In 2008, North Korea ceremoniously blew up the reactor’s cooling tower in a largely made-for-TV event amid nuclear talks between the United States and then-leader Kim Jong Il. (A new cooling tower was built after the negotiations fell through.)

Leading up to the July restart of the 5-megawatt reactor, a steam plant at a radiochemical laboratory was in operation for five months beginning in mid-February, according to the report. This stood out because the duration is consistent with the time required to process a full batch of fuel from the reactor and much longer than is needed for maintenance or waste management, the report said.

The continuation of North Korea’s nuclear program “is a clear violation of relevant UN Security Council resolutions and is deeply regrettable,” the report said.

A senior Biden administration official, speaking on the condition of anonymity to discuss sensitive matters, said U.S. officials are aware of the IAEA report.

“This report underscores the urgent need for dialogue and diplomacy so we can achieve the complete denuclearization of the Korean Peninsula. We continue to seek dialogue with the DPRK so we can address this reported activity and the full range of issues related to denuclearization,” the official said in a statement. DPRK refers to North Korea’s official name, the Democratic People’s Republic of Korea.

IAEA inspectors have not had access to North Korea since 2009, when they were kicked out. The agency has been using open-source information, satellite imagery and other material to monitor developments.

Jeffrey Lewis, director of the East Asia Nonproliferation Program at the James Martin Center for ­Nonproliferation Studies in Monterey, Calif., said that while other open-source analysts have made the same observations, the IAEA’s report highlights the ways in which North Korea’s nuclear program has continued even in the absence of weapons testing. North Korea’s last nuclear test was in 2017.

“People ignore North Korea’s nuclear program when it isn’t conducting nuclear explosions,” he said. “But even though North Korea stopped testing, it never stopped making new nuclear weapons.”

Also in July, North Korea agreed to restore a key communications hotline with South Korea to improve cross-border ties. After that announcement, however, North Korea remained unresponsive to calls on the hotline as the United States and South Korea held joint military exercises.

Officials from the North Korean mission to the United Nations did not immediately respond to a request for comment on Monday.

Lee Jong-joo, spokeswoman for Seoul’s Unification Ministry, said in a news briefing on Monday that the South Korean government is monitoring the situation closely in cooperation with the United States.

“Based on this, we will make continued efforts for peace and complete denuclearization of the Korean Peninsula, and development of inter-Korean relations,” Lee said. “However, we have nothing to confirm regarding intelligence matters including signs of North Korea's nuclear activity.”

Min Joo Kim in Seoul contributed to this report.