US military officials say an American soldier crossed the demilitarized zone that separates North and South Korea on Tuesday. They say the man acted "willfully and without authorization."

The officials say the soldier was on a civilian tour of Panmunjom. Tourists get the opportunity to watch North Korean soldiers up close. The officials did not say why he crossed, but they say authorities in North Korea took him into custody.

US Defense Secretary Lloyd Austin said, "I am absolutely foremost concerned about the welfare of our troop. And so we will remain focused on this and again this will develop in the next several days."

Media outlets say US officials have confirmed the soldier had been released from a military prison in South Korea. They added that he was to face disciplinary action back in the US.

Officials at the State Department advise Americans not to travel to North Korea. They warn there is a "serious risk of arrest and long-term detention."