Quoting the "informed source," the South Korean news agency reported Wednesday the North, whose activities this week included claims of a nuclear test and firings of short-range missiles, apparently has resumed work at Yongbyon to produce plutonium.
The communist nation was to have disabled the facility in return for foreign aid under the now-stalled six-nation talks.
The source told Yonghap Yongbyon might have been restarted in mid-April.
North Korea had said in April it would restart it after the U.N. Security Council condemned its earlier long-range rocket launch.
The latest development comes amid growing international furor over the North's claims Monday of a second nuclear test since October 2006, and firings short-range missiles Tuesday.
The source told Yonhap the doors of the Yongbyon facility holding spent fuel rods were opened several times in April and that since then plumes of steam were noticed at a reprocessing plant designed to use these rods to produce plutonium.
"Since early May, we have continuously been seeing steam rising from the nuclear fuel fabrication plant," the source was quoted as saying.
It is "the belief of our intelligence office that it is highly likely the North restarted its nuclear facility," the source said.