Defense Secretary Lloyd Austin on Tuesday confirmed that a U.S. soldier being detained by North Korea “willfully and without authorization” crossed the border from South Korea into the pariah state.

“We’re very early in this event, and so there’s a lot that that we’re still trying to learn, but what we do know is that one of our service members who was on a tour willfully and without authorization crossed the military demarcation line,” Austin told reporters at the Pentagon.

The soldier is believed to be in North Korean custody, and U.S. officials are “closely monitoring and investigating the situation and working to notify the soldier’s next of kin and engaging to address this incident,” Austin added.

“I’m absolutely foremost concerned about the welfare of our troop. And so we will remain focused on this,” he said.

Multiple reports emerged earlier Tuesday that a junior enlisted U.S. soldier crossed into North Korea after facing disciplinary action by the U.S. military.

Col. Isaac Taylor of U.S. Forces Korea Public Affairs confirmed in a statement that the troop had been on a civilian tour of the Demilitarized Zone (DMZ), a buffer space between North and South Korea, and was not on duty at the time. The soldier broke from the group and crossed the demarcation line, he said.

Taylor added that the U.S. is working with the Korean People’s Army, North Korea’s military, to “resolve this incident.”

The soldier, reportedly identified as Private 2nd Class Travis King, is said to have parents located in the United States.

Biden press secretary Karine Jean-Pierre said later Tuesday that the “primary concern is ascertaining the well-being of this individual” and the White House is looking into the situation and trying to get more information.

Jean-Pierre said she had no calls to read out from President Biden to either North Korea or South Korea, but said the U.S. is “engaging” with Sweden and South Korea on the issue.

House Armed Services Committee ranking member Adam Smith (D-Wash.) told CNN that the incident “creates a significant diplomatic problem” between Pyongyang and Washington.

“The first step is going to be reestablishing those communications, but if a U.S. soldier is in North Korean custody, we need to do what we can to get him back,” Smith said.

The crossing of the U.S. soldier into North Korea is likely to further raise tensions between the two countries currently in conflict over the Pyongyang’s nuclear weapons provocations and continued missile launches, as well as America’s military footprint in the Indo-Pacific.

Also on Tuesday, the USS Kentucky, a nuclear-armed submarine, arrived in Busan, South Korea, for a scheduled port visit — the first time such a vessel has been to the country since the 1980s.

Seoul and Washington in April agreed to periodic visits by U.S. nuclear ballistic missile-capable submarines to South Korea, a response to North Korea’s continued nuclear threat.