A U.S. think tank says a recent satellite photo shows North Korea has resumed operating a five-megawatt nuclear reactor in Yongbyon, some 90 kilometers north of Pyongyang.

The Institute for Science and International Security on Wednesday said the photo taken Sunday and posted on its homepage shows steam coming from the reactor's cooling tower at the North's main nuclear site.

Another photo could indicate new activity, possibly the North's resumption of building a 50-megawatt reactor.

Pyongyang in April said it suspended work at the plant used to extract spent nuclear fuel, but the Japanese daily Asahi Shimbun last month said the reactor resumed operations in July.