(Bloomberg) -- North Korea appears to have resumed operations at its Yongbyon nuclear reactor that produces plutonium, a key material used to make nuclear weapons, the Wall Street Journal reported, citing a report by the U.N. atomic agency.

The move would create an additional foreign policy challenge for the Biden administration, whose nuclear envoy visited Seoul last week amid threats of a “security crisis” from Pyongyang. The Yongbyon reactor appeared to have been inactive from December 2018 until the beginning of July this year, the Journal cited the report as saying. 

“Since early July, there have been indications, including the discharge of cooling water, consistent with the operation of the reactor,” according to the annual report by the International Atomic Energy Agency.

Yongbyon, located about 100 kilometers (60 miles) north of the capital, has been regarded as the crown jewel of North Korea’s nuclear weapons program. First constructed in 1979, its reactor supplied the plutonium and research facilities needed for North Korea to test its first atomic bomb in 2006.

Last Sunday, Sung Kim, the U.S. special representative for North Korea, said the U.S. has no hostile intent toward North Korea, and the ongoing U.S.-South Korea military exercises are routine and defensive. 

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