The reactor at North Korea's Yongbyon nuclear complex continues to show signs of operation, suggesting that the country may be resuming the production of weapons-grade plutonium, a U.S. institute monitoring the site said Monday.

Steam vapor plumes were seen emanating from the 5 megawatt reactor facility and nearby river ice was melting, based on commercial satellite imagery from Feb. 25, the U.S.-Korea Institute of Johns Hopkins University said on its 38 North website.

(Photo supplied by Airbus Defense & Space/38 North)

The findings could overshadow a recent diplomatic thaw between the two Koreas.

"While vapor plumes have been noted a few times over the past year, no cooling water discharges have been observed to support this conclusion (on the reactor's operation). However, we cannot rule out that the North Koreans may have suppressed this signature by extending the discharge pipe into the river," it said.

"If the reactor is operating again, as the evidence suggests, it means North Korea has resumed production of plutonium presumably for its nuclear weapons program," it added.

In the southern parts of Yongbyon, a military tent camp has also been set up and increased personnel have been observed around the complex, the website said.

"It is unclear what specific role this military camp will play, but it could be to support new construction and/or improve site security," it said.