Pyongyang, North Korea - The US Department of Defense has revealed new details about the case of an American soldier who crossed into North Korea without permission on Tuesday.

The man had been escorted to the airport but disappeared before boarding the plane, Pentagon spokesperson Sabrina Singh said in Washington on Thursday.

The soldier had completed his deployment and should have returned home after spending time in a South Korean detention center, Singh said. He had been threatened with "additional administrative measures" in the US. She did not give more details.

She also did not give details about the background of his detention.

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South Korean news agency Yonhap reported on Wednesday, citing judicial authorities, that US Private Travis King had refused to pay fines for kicking and damaging a police car in Seoul.

Singh said that King was not in custody on the way to the airport. However, he had escorts who took him through security.

"I don't think anyone anticipated that he was going to leave the airport," she said.

Singh said the man joined a commercial tour of the border, where he broke away from the group and crossed the demilitarized zone (DMZ) that separates the two Korean nations, who are still officially at war since a conflict in the 1950s.

"We don't know what he did in the hours between when he left the airport and when he crossed over into the DPRK (North Korea)," Singh said.

The US has had no word from North Korea on where King was being held or on his condition, she said.

The aim is to bring the US soldier home, Singh said.