SEOUL/WASHINGTON (Reuters) -An American soldier facing disciplinary action fled across the inter-Korean border into North Korea on Tuesday and was believed to be in North Korean custody, U.S. officials said, creating a fresh crisis for Washington in its dealings with the nuclear-armed state. Defense Secretary Lloyd Austin expressed concern for the soldier, who the U.S. military in Korea said joined an orientation tour of Joint Security Area between the Koreas and "wilfully and without authorization crossed the Military Demarcation Line into the Democratic People’s Republic of Korea (DPRK)."