North Korea has reportedly restarted its nuclear reprocessing facility at Yongbyon.

A diplomatic source in Seoul said on Wednesday the doors of a storehouse where the North holds spent fuel rods for plutonium production were opened "several times" since mid-April, which was when the North announced it would restart nuclear reprocessing.

The source also said that plumes of steam were seen emanating from a fuel reprocessing plant starting in late April.

Since mid-May, movements of vehicles suspected of carrying nitric acid --- a key ingredient in the extraction of plutonium from nuclear fuel --- were also detected around the Yongbyon nuclear fuel plant.

The source said that given the information, South Korean intelligence authorities believe it is highly likely the North has restarted its nuclear facility.

The source added that intelligence cooperation between Seoul and Washington on the matter is proceeding smoothly.