Recent satellite imagery shows "suspicious activity" at North Korea's nuclear complex in what could be a sign that Pyongyang might be trying to harvest weapons-grade plutonium from spent nuclear fuel, a U.S. website monitoring the North said Monday.

The website 38 North said in a report that the satellite imagery has shown "exhaust plumes" from a thermal plant used to heat the Yongbyon nuclear complex's Radiochemical Laboratory, where spent nuclear fuel rods are reprocessed to extract plutonium.

"During the past five weeks, exhaust plumes on two, possibly three, occasions were observed at the Radiochemical Laboratory's Thermal Plant," the report said, adding that the activity is "unusual" since exhaust plumes have rarely been seen there and none has been observed on any examined imagery this past winter.

"The plumes suggest that the operators of the reprocessing facility are heating their buildings, perhaps indicating that some significant activity is being undertaken, or will be in the near future," it said.

It remains unclear whether the activity will be additional separation of plutonium, the report said.

But last month, Director of National Intelligence James Clapper said in a worldwide threat assessment report that the North had restarted its five-megawatt reactor at Yongbyon and has since run it for long enough to harvest plutonium "within a matter of weeks to months."

The graphite-moderated reactor has been the source of weapons-grade plutonium for the communist nation. The small reactor is capable of producing spent fuel rods that, if reprocessed, could give the regime enough plutonium to make one bomb a year.

The reactor has provided Pyongyang with weapons-grade plutonium that the regime used in its first three nuclear tests, in 2006, 2009 and 2013. The North conducted its fourth nuclear test on Jan. 6, claiming it successfully detonated a hydrogen bomb.

Meanwhile, 38 North also said that the North is making progress in construction of a light water nuclear reactor at the Yongbyon complex, though it is still unclear whether the reactor will become operational within this year.

Should the reactor become operational, it could serve as a new source of fissile material for nuclear weapons.

Assistant Secretary of State Daniel Russel declined to comment on the amount of fissile material in the North's possession, citing intelligence matters. But he stressed that neither nuclear materials nor missiles would make the regime more secure.

"There's no amount of fissile material that will improve North Korea's strategic position. There is no improvement to its delivery system that will make North Korea safer, or provide for its security," Russel said during a seminar hosted by the Institute for Corean-American Studies.

"The notion that the DPRK is deterring some malevolent hostile power that's bent on its destruction is absurd. It is not borne out by reality and I've ventured to argue that not even the DPRK's leadership really believes it," he said. (Yonhap)