The U.S. State Department is renewing its call for North Korea to release a U.S. Christian missionary hospitalized in Pyongyang after being sentenced to 15 years at a hard labor camp.

Kenneth Bae, an American citizen, was arrested on Nov. 3, 2012, in the port city of Rajin and sentenced in May to 15 years of hard labor for committing "hostile acts" and "plotting to overthrow the government," according to his sister, Terri Chung, who spoke to FoxNews.com.

Bae, who was sent to a special labor camp where he was the only prisoner, was hospitalized after three months when his health started to fail, Chung said, adding that her 45-year-old brother suffers from diabetes and an enlarged heart, among other medical conditions. Bae's mother, Myunghee Bae, was granted a five-day visa to North Korea last month to visit her son for a total of six hours.

On Tuesday, State Department Deputy Spokesperson Marie Harf told FoxNews.com that, "We remain gravely concerned about Mr. Bae’s health and we continue to urge the DPRK authorities to grant Mr. Bae special amnesty and immediate release on humanitarian grounds. We are in regular contact with the Bae family and helped facilitate communications with our protecting power, the Government of Sweden, for Mrs. Bae’s private trip."

"We continue to work actively to secure Mr. Bae’s release, working in close consultation with the Swedish Embassy," Harf said. "If the DPRK renews its invitation, Special Envoy for North Korean Human Rights Ambassador Robert King is prepared to travel to the DPRK on a humanitarian mission focused on securing the release of Mr. Bae."

Bae, who has a wife and three children, is a Christian missionary who was based in China and working as a tour guide at the time of his arrest. He has been held longer by the North Korean regime than any other known U.S. citizen since the Korean War, according to International Christian Concern, a Washington, D.C.-based activist group that focuses on the human rights of Christians.

"Sunday we marked with great sadness the one-year anniversary of Kenneth Bae’s detainment," Isaac Six, the group's advocacy manager, said in a statement to FoxNews.com.

"No one should ever face imprisonment or this type of treatment simply because of their religious beliefs, yet thousands upon thousands of men, women, and children are living in what has been described as hell on earth simply because they choose to be Christian," Six said. "Our hope is that the world takes this highly unfortunate anniversary as an opportunity to wake up to what is happening behind the closed walls of North Korea and to call with one voice for an end to the atrocities being committed by Kim Jong-uns' malignant regime."

Chung said her family's worst fear is that Bae will be sent back to the labor camp for the next 15 years -- a sentence the North Korean regime considered lenient given his "crime."

"He's a Christian and he has strong convictions ... I’m afraid that was interpreted as anti-government," Chung said. "We are pleading with North Korean leaders to show amnesty and mercy."