The family of an American missionary who has been detained for almost two years in North Korea expressed their worry for his health on Monday.

Accused of attempting to overthrow the secretive nation, Kenneth Bae, a 46-year-old American tour guide and missionary was detained in North Korea and sentenced to 15 years in a labor camp in 2012.

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On Monday Bae, together with two others, Jeffrey Fowle and Matthew Miller, was able to air his grievances and issue a plea to the U.S. government to send a high-ranking negotiator to appeal for their release.

During the press conference with the Associated Press, Bae's sister, Terri Chung, observed how her brother was obviously in pain while conducting the interview.

She noted a huge difference in Bae's demeanor as compared to the usually cheerful man she used to know.

"As an incredibly friendly and social man, the psychological pain of isolation and worry for his family must weigh on him as much as the physical agony," she said in a statement.

Chung pleaded with the North Korean government to release his brother, citing that he has confessed to the crime and has already served an extensive punishement.

Since November 2012, Bae has been serving his sentence at a labor camp where he does manual labor eight hours a day.

Echoing his brother and two other American detainees' plea, Chung called on the American government to send help in expediting their release.

In response, Spokesperson Patrick Ventrell of the National Security Council assured that the U.S. government is working on convincing the country to release the U.S. citizens, citing that they are putting the issue at the top of their priority list.

He also vowed that the U.S. government is doing everything in its power to "secure their earliest possible release."

TagsNorth Korea, Kenneth Bae, hard labor camp, north korea prison sentence, missionary