WASHINGTON: US Defense Secretary Lloyd Austin expressed concern on Tuesday for a soldier who is believed to be in North Korean custody after "willfully" crossing into the country from South Korea, and said the US military was still working to notify his family.

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"There's a lot that we're still trying to learn," Austin told a news briefing. "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."