Updated

A U.S.-based institute says new satellite imagery shows that North Korea has resumed building work on a reactor after months of inactivity.

That indicates the North is pressing on with efforts to expand its nuclear program, the institute says, despite international criticism. North Korea says the reactor is intended to generate electricity but its active pursuit of nuclear weapons raises doubts over its intentions.

The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies said Wednesday the image from a commercial satellite dated April 30 shows progress in construction of the containment building for the light-water reactor at the North's main nuclear facility at Yongbyon.

But the institute says the reactor is unlikely to become operational before 2014 or 2015.

Pyongyang expelled U.N. nuclear monitors three years ago.