SEOUL, South Korea — North Korea is expanding an important missile base that would be one of the most likely sites for deploying intercontinental ballistic missiles capable of reaching the United States, two experts on the North’s missile programs said Thursday, citing new research based on satellite imagery.

The activities at the Yeongjeo-dong missile base near North Korea’s border with China and the expansion of a new suspected missile facility seven miles away are the latest indications that North Korea is continuing to improve its missile capabilities, said Jeffrey Lewis and David Schmerler of the Middlebury Institute of International Studies at Monterey in California. And they come despite President Trump’s repeated claims of progress in efforts to denuclearize the North.

Dr. Lewis and Mr. Schmerler said they were still not sure whether Yeongjeo-dong and the new facility under construction in nearby Hoejung-ni, both in the mountainous area near North Korea’s central border with China, were separate bases or parts of a larger single operation.

But their geographic locations make them ideal to “house long-range missiles,” they said in a report they were preparing.