SEOUL, June 8 -- South Korea's unification ministry in charge of inter-Korean affairs on Wednesday expressed deep worry about a report that the Democratic People's Republic of Korea (DPRK) had restarted plutonium reprocessing activities at its main nuclear complex.

Unification Ministry spokesman Jeong Joon Hee told a press briefing that it was well-known for the DPRK to have pursued various reprocessing activities to secure additional plutonium, noting that South Korea has been closely monitoring relevant situations with a serious concern.

Jeong added that close cooperation between South Korea and the United Stateshas been maintained in regards to the matter.

His comments came after reports that Pyongyang had restarted production of plutonium fuel citing an unidentified senior U.S. State Department official.

According to the official, the DPRK took spent fuel in the past from the 5-megawatt reactor at Yongbyon to take it to the reprocessing facility. The same activities are happening in Yongbyon, a city some 100 km northeast of its capital Pyongyang.

Pyongyang shut down the graphite-moderated reactor at Yongbyon in 2007 in accordance with agreements reached at the six-party talks, exploding the cooling tower in June the next year.

The DPRK, however, declared the resumption of the 5-megawatt reactor operations in April 2013, right after adopting the so-called simultaneous developments in economy and nuclear weapons.

The disarmament-for-aid talks to dismantle the DPRK's nuclear program, which involve South Korea, the DPRK, China, the United States, Russiaand Japan, have been halted since late 2008.