North Korea appears to have doubled the size of the area used to enrich uranium at its Yongbyon reactor complex in recent months, a proliferation monitoring group reported Wednesday, raising new concerns that the country could increase production of weapons-grade fuel — even as it says it wants to relax tensions with South Korea and the United States.

The monitoring group, the Institute for Science and International Security in Washington, said its calculation was based on comparative satellite imagery of the Yongbyon complex. The uranium-enrichment building, in an image taken on June 10, showed an expansion of roughly the same length and width as the original size of the building, from construction that apparently had begun in March, the institute said in a study posted on its Web site.

That means the expansion would have begun shortly before North Korea announced in April that it planned to restart a mothballed nuclear reactor at Yongbyon and that it intended to use the uranium enrichment facilities there to make weapons. The announcement came when tensions with South Korea and the United States were escalating in the aftermath of the North’s third nuclear test. Previously, North Korea had insisted the Yongbyon plant was for only civilian energy purposes.

“This announcement may have been partially intended as an oblique effort to reveal this new construction, one missed publicly at the time,” wrote the authors of the satellite study, David Albright and Robert Avagyan.