• The Pentagon confirmed it was aware that a U.S. servicemember was in North Korean custody.
• The U.S. servicemember "willfully" and "without authorization" crossed the inter-Korean border.
• The identity of the servicemember will be released after next of kin are notified.

WASHINGTON — The Pentagon on Tuesday said it was aware that a U.S. servicemember "willfully" and "without authorization" crossed the inter-Korean border and is currently in North Korean custody.

U.S. authorities are working with North Korea's military "to resolve this incident," U.S. Forces Korea spokesman Col. Isaac Taylor said in a statement.

The person was touring the Joint Security Area, or JSA, a site at Panmunjom in the Demilitarized Zone used by North and South Korea for diplomatic engagements and negotiations. It is bisected by the Military Demarcation Line, also known as the Armistice Line, which acts as the border between the two territories. Both private companies and the United Nations organize tours of the JSA.

The identity of the servicemember will be released after next of kin are notified.

A State Department spokesperson referred questions to the Pentagon. The spokesperson said the State Department was "in touch with counterparts at the Department of Defense to provide all appropriate assistance."

The White House did not immediately respond to CNBC's request for comment.

The U.S. and North Korea do not currently have formal diplomatic relations. The State Department advises U.S. nationals not to travel to North Korea "due to the continuing serious risk of arrest and long-term detention."

The incident takes place on the same day a U.S. nuclear submarine visited South Korea for the first time since the 1980s, arriving at the port of Busan, according to the U.S. forces in South Korea.

Ruxandra Iordache contributed reporting from CNBC International's bureau in London.