WASHINGTON, July 19 (Reuters) - The U.S. military was scrambling to establish the fate of an American soldier who made an unauthorised crossing of the inter-Korean border into North Korea, throwing Washington into a new crisis in its dealing with the nuclear-armed state.

The U.S. Army identified the soldier as Private Travis T. King who joined up in 2021 and was facing disciplinary action. While on a orientation tour of Joint Security Area (JSA) on the border between the two Koreas, King crossed into North Korea on Tuesday "wilfully and without authorization," U.S. Defence Secretary Lloyd Austin said.

"We believe that he is in (North Korean) custody and so we're closely monitoring and investigating the situation and working to notify the soldier's next of kin," Austin told a briefing.

North Korea's state media have made no mention of the incident. Its mission to the United Nations in New York did not immediately respond to requests for comment.

The crossing comes at a time of renewed tension on the Korean peninsula, with the arrival of a U.S. nuclear-armed ballistic missile submarine, and the launch early on Wednesday morning of two ballistic missiles into the sea by North Korea.

North Korea has been testing increasingly powerful missiles capable of carrying nuclear warheads, including a new solid-fuel intercontinental ballistic missile launched last week.