SEOUL, South Korea — North Korea has resumed construction of a nuclear reactor that can be used to expand the country’s nuclear weapons program, an American-based institute said Thursday, citing the latest satellite imagery of the building site.

In November, North Korea reported brisk progress in the building of a small light water reactor in its main nuclear complex in Yongbyon, north of Pyongyang, its capital. If completed and operational, the plant would give North Korea a new source of spent nuclear fuel from which plutonium, a fuel for nuclear weapons, can be extracted.

North Korea also unveiled a uranium enrichment plant in Yongbyon in November 2010, saying that it was enriching uranium for fuel for the reactors it planned to build to resolve its electricity shortages. But international nuclear experts believed that the North’s enrichment program was also intended to produce highly enriched uranium, another type of fuel for nuclear bombs.

Recent commercial satellite imagery, including photography taken on April 30, shows that North Korea has resumed building work in Yongbyon after months of inactivity and that the country is close to completing the reactor containment building, according to an analysis posted on Thursday at 38 North, a Web site run by the U.S.-Korea Institute at Johns Hopkins University.