South Korea was placed on a U.S. Department of Energy watchlist because visitors to its labs mishandled sensitive information, Joseph Yun, acting U.S. ambassador to South Korea, said Tuesday.

The U.S. Department of Energy confirmed this week that it had designated South Korea a "sensitive" country in January, but did not explain why.

South Korea's acting President Choi Sang-mok was to be briefed on Tuesday by vice ministers on their response to being listed, and Industry Minister Ahn Duk-geun is expected to ask the U.S. Energy Secretary to remove South Korea from the list of sensitive countries during his visit to the U.S. this week, according to government sources.