A U.S. national believed to have been detained by North Korea after crossing the border from the South is thought to be an American soldier, a U.S. official familiar with the situation told NBC News on Tuesday.

The source added that U.S. officials were still gathering information on the “fluid” incident, which comes amid heightened tensions over North Korea’s nuclear program.

Earlier, the American-led U.N. Command said a U.S. national on an orientation tour had “crossed, without authorization, the Military Demarcation Line into the Democratic People’s Republic of Korea.”

“We believe he is currently in DPRK custody and are working with our KPA counterparts to resolve this incident,” it said in a tweet.

NBC News has reached out to the White House for comment.

The unnamed person was on a tour of the joint security area, part of the demilitarized zone between the two countries, according to the United Nations.

Tours of the area have been ongoing since the 1960s and are organized by private companies, while the U.N. has held its own tours for its staff.

The area is just 30 miles north of Seoul, the South Korean capital, and has huge historic importance as the place where the armistice to end the Korean War was signed in 1953.

The news comes as the USS Kentucky, a nuclear-powered submarine, arrived at the Busan naval base in South Korea on Tuesday.

“This port visit to Busan reflects the United States’ ironclad commitment to the Republic of Korea for our extended deterrence guarantee,” U.S. Forces Korea said in a statement.

“This is a developing story. Please check back for updates.”

This article was originally published on NBCNews.com

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