WASHINGTON 

The US has delivered messages to North Korea, indicating that American soldier Pvt. Travis King had crossed the border on his own and requested more information about his well-being, said State Department on Thursday.

"We have relayed messages to the DPRK (Democratic People's Republic of Korea) that private King crossed on his own and we want him to return safely," Spokesman Matthew Miller told reporters during a daily press briefing.

"We have also asked for more info on his well-being. I will say that we retain a number of channels through which we can send messages to DPRK," he added.

Miller has not shared further details, saying that "those discussions are quite sensitive".

"But we remain engaged in this matter. It is a priority for us to seek a safe return," he continued.

King remains in North Korean custody after crossing the heavily militarized border separating the Koreas on Sunday. He was reportedly on his way back to the US to face disciplinary action after spending time in a South Korean military prison on assault charges.