The deputy commander of the UN Command in South Korea says the body has been in contact with the North over a US soldier who crossed into the country last Tuesday.

Lieutenant General Andrew Harrison told foreign media on Monday that the UN command and the North Korean military have been in touch over the matter.

The consultations are taking place by means of communication lines in place in the Joint Security Area that straddles the demarcation between the two Koreas and is overseen by the UN Command.

He also said the primary concern is the wellbeing of the soldier, who has been identified as Private Travis King.

Harrison declined to give details, but said he remained optimistic despite the fact that "there was no way to know how the conversation with North Korea would proceed."

South Korean media say that Private King belongs to the US forces stationed in the country. He was reportedly detained after causing trouble and was about to be sent back to the US to face disciplinary action.

Instead, he fled the airport and took part in a civilian tour of the demilitarized zone between the two Koreas, which encompasses the Joint Security Area.

Experts in South Korea suggest that the North may use the US soldier to carry out so-called "hostage diplomacy," but Pyongyang has so far given no indication of its intentions.