North Korea seems to be notably expanding activities at its main nuclear complex in Yongbyon, a U.S. think tank said Thursday, based on analysis of new satellite imagery.

The revelation adds to worries that the communist nation may be turning back to a provocation cycle after months of unanswered peace overtures towards Washington.

"Recent commercial satellite imagery shows that North Korea's Yongbyon nuclear center appears to be increasingly active. There are several signs of new and continued activity and progress in the construction of facilities," the Institute for Science and International Security (ISIS) said in a report.

It cited commercial satellite photos taken Monday that show steam is present at the fuel fabrication complex.

"The steam is emerging from a building that years ago was involved in converting yellowcake to natural uranium dioxide," the Washington-based institute said. "One implication of the steam is that the building is operational as part of an effort to make fuel for the 5 MWe reactor."

ISIS raised the possibility that the North's uranium enrichment program is also making progress.

"Since earlier this year, North Korea has been expanding its gas centrifuge enrichment plant at Yongbyon," it said. "This plant is believed to be producing low enriched uranium fuel for the light water reactor (LWR) or for further enrichment to weapon-grade at Yongbyon or elsewhere."

Last month, the International Atomic Energy Agency announced it also detected the release of steam and water from the Yongbyon reactor in an indication that it has been restarted. (Yonhap News)