Kenneth Bae's sister is urging US leaders to secure her brother's release from a North Korean prison.

Bae's sister, Terri Chung, was responding to despondent comments attributed to her brother in an interview published Wednesday, Fox reports.

The interview was published in a pro-North Korean, Japanese newspaper called Choson Sinbo. The paper quoted Bae as saying that he feels that his case is not progressing, and he feels "abandoned by the United States government".

North Korea has held Korean-American Kenneth Bae in prison for over 20 months—the longest North Korean imprisonment of an American since the Korean War. Bae was working in the country as a tour guide, but North Korea alleges that Bae's North Korean tour company was a front for Christian evangelical missions.

Bae, 46, was consequently sentenced to 15 years hard labour in April 2013.

Chung said that the family had not heard from the devout Christian since April this year, and were saddened to read the interview.

"After months of silence, it is devastating to hear Kenneth talk about 'feeling abandoned by the United States government'," she said.

"Although we acknowledge and appreciate all the efforts the US State Department has been making behind the scenes to secure Kenneth's release, the fact remains that after almost 2 years, Kenneth still remains imprisoned in North Korea.

"When we woke this morning to hear news about Kenneth, we joined him in feeling the disappointment, wondering when this will ever end."

Former US Ambassador to the United Nations Bill Richardson was unsuccessful in securing Bae's release, but State Department spokeswoman Marie Harf said the administration is still trying to secure Bae's release.

"We are very concerned about his health," she told reporters on Thursday.

"We have urged the [Democratic People's Republic of Korea] authorities to grant him special amnesty and immediate release on humanitarian grounds."

She also warned not to take the published statements attributed to Bae to heart.

"I would just take with a grain of salt things people say in videos when they are being held by a country like North Korea," Harf said.

Chung said she just wants her brother back home.

"With Kenneth's health continuing to deteriorate, we cannot afford to wait any longer," she said in a plea to President Obama and Secretary of State John Kerry.

"Please do whatever it takes to bring Kenneth home."