Acting U.S. Ambassador to South Korea Joseph Yun said Tuesday that Seoul was added to the U.S. Department of Energy’s list of sensitive countries because of cases of mishandling sensitive information, stressing that it is not a matter to be overly concerned about.

During a panel discussion co-hosted by the American Chamber of Commerce in Korea and the U.S. Embassy in Seoul, Yun expressed regret over the situation.

He explained that the sensitive country list is limited to the U.S. Department of Energy’s laboratories, noting that multiple facilities are under the department.

Yun said more than two-thousand South Korean students, researchers and public officials had visited research labs containing sensitive materials and there had been some incidents.

The acting ambassador confirmed that the country was placed on the list due to carelessness in handling sensitive information, but he did not specify who was responsible.