U.S. diplomats have been meeting with the top North Korean nuclear negotiator aiming to free American prisoners and to limit North Korea’s missile and nuclear programs, the Wall Street Journal reported Sunday.

North Korean negotiator Choi Sun Hee has been working with the U.S. for a long time, meeting with diplomats from the administrations of former Presidents Bill Clinton and George W. Bush. Former President Barack Obama had a much more limited engagement with the North Korean regime, employing a policy called “strategic patience.”

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The contact began with back channels more than a year ago, the U.S. has no official diplomatic relationship with North Korea. The meetings also helped secure the release of an American prisoner in June, 22-year-old Otto Warmbier. The University of Virginia student was arrested in Pyongyang, North Korea’s capital, in January 2016. Warmbier was in the country before beginning a study abroad program in China and was charged with hostile acts against the state for allegedly tampering with a propaganda poster. He was charged with 15 years of hard labor.

At some point in his detainment, Warmbier slipped into a coma and has appeared to be in a coma for at least a year. At a press conference after his release, Warmbier’s medical team at the University of Cincinnati said he had suffered severe neurological damage, most likely from a cardiopulmonary arrest. The damage to his brain most likely came from lack of oxygen.

RTX38PBF North Korean leader Kim Jong Un inspects the long-range strategic ballistic rocket Hwasong-12 (Mars-12) in this undated photo released by North Korea's Korean Central News Agency, May 15, 2017. Photo: KCNA/REUTERS

The condition that Warmbier was in could cause some problems for any ongoing talks between the U.S. and North Korea.

“Given the reported status of Mr. Warmbier’s condition, any diplomatic path forward is going to be extremely difficult,” Suzanne DiMaggio of the New America Foundation told the Journal. “If the North Koreans immediately released the remaining three prisoners, it could set up an atmosphere for potentially serious talks.”

The New America Foundation is a think tank in Washington and DiMaggio helped set up the latest talks with North Korea.

The three prisoners left in North Korea are Kim Hak Song and Tony Kim, both university professors, and Kim Dong Chul a businessman.

Former New Mexico Gov. Bill Richardson has been conducting back-channel meetings of his own with North Korea, mostly aimed and ensuring Warmbier’s release.

“He’s in a coma and the North Koreans have a lot of explaining to do especially since he might have been in a coma for a year,” Richardson told WTOP, Washington, Thursday. “We’ve been meeting with the North Koreans, urging the release. We offered some humanitarian assistance flood assistance, paying for the remains of our soldiers and the North Korean’s never responded.”

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The first official contact North Korea had with the administration of President Donald Trump was with the State Department’s Joseph Yun, the Journal reported. The meeting didn’t happen until the beginning of June when the U.S. finally learned Warmbier was in a coma. An earlier meeting was called off due to the assassination of North Korean leader Kim Jong Un’s half-brother in Malaysia earlier this year. U.S. officials have said they believe the killing was state-sanctioned.

North Korean officials maintain they will not abandon their nuclear program altogether but may be open to limiting its scope or temporary halts, the Journal reported.