WASHINGTON D.C. (Reuters) - A U.S. missionary being held in North Korea was moved from a hospital back to a labor camp last month on the same day he made a public appeal for Washington to help get him home, the U.S. State Department said on Friday, citing Swedish diplomats who met the prisoner.

Kenneth Bae, 45, has been held for more than a year in North Korea after being sentenced to 15 years of hard labor for trying to overthrow the state. From last summer until Jan. 20, he had been kept at Friendship Hospital in Pyongyang, the North Korean capital, his family said.

"The Department of State has learned that the DPRK transferred Mr. Bae from a hospital to a labor camp, a development with which we are deeply concerned," State Department spokeswoman Jen Psaki said.

"We also remain gravely concerned about Mr. Bae's health, and we continue to urge DPRK authorities to grant Mr Bae special amnesty and immediate release on humanitarian grounds," she said, referring to North Korea by the acronym of its official name, the Democratic People's Republic of Korea.

Psaki said Swedish Embassy representatives had met Bae 10 times since his detention, most recently on Friday in a labor camp.

"We continue to work actively to secure Mr. Bae's release," Psaki said, adding that Washington remained prepared to send its human rights envoy for North Korea, Robert King, to Pyongyang for that purpose.

North Korea has rejected this offer in the past and withdrew an invitation for King to visit Pyongyang last August.

Bae said in an interview with a pro-North Korea newspaper published in Japan that a Swedish Embassy official had visited him on Friday and told him King would visit as early as Monday and by the end of the month at the latest.

The United States had offered to send civil rights activist Jessie Jackson but North Korea has instead approved the visit by King, Bae said in the interview with the Choson Sinbo newspaper issued on Friday. It did not have further details on King or Jackson's plans.

A State Department official said Bae was moved back to the labor camp on Jan. 20.

Bae's sister, Terri Chung, told Reuters Bae had been held in a labor camp from May 14 last year until Aug. 5, when he was moved to the hospital.

She said the family did not know where the camp was, except that it was far from Pyongyang and Bae was working eight hours a day, six days a week.

HEALTH WORRY

Chung said her brother suffered from a variety of health issues, including diabetes, an enlarged heart, kidney stones and severe back pain.

"We are very concerned about his health," she said.

Bae, a Korean American, last appeared in public at Pyongyang Friendship Hospital on Jan. 20 when he was paraded in front of a group of reporters and asked Washington to help him get home.

Bae's media appearance was his second since his arrest in 2012 when he led a tour group into the country. North Korea's state KCNA news agency reported Bae himself had asked to hold the news conference.

Bae has acknowledged being a missionary and has said he conducted religious services in the North, one of the most isolated states on earth and long hostile to Westerners advocating religious causes.

On Thursday, President Barack Obama offered prayers for Bae and U.S. prisoners held in other countries during remarks at an annual prayer breakfast that highlighted his Christian faith.

"His family wants him home. And the United States will continue to do everything in our power to secure his release," Obama said.

On Tuesday, the last surviving members of the U.S. Congress to have served in the Korean War sent a letter to North Korean leader Kim Jong-un asking him to release Bae.

North Korea in December released 85-year-old Korean War veteran Merrill E. Newman, a former U.S. special forces soldier who had been held since October after visiting the country as a tourist, and the members of Congress applauded that in the letter seeking Bae's freedom.

The letter, signed by Democratic Representative Charles Rangel from New York, Democratic Representative John Conyers Jr. from Michigan, Republican Representative Sam Johnson from Texas and Republican Representative Howard Coble from North Carolina, is not seen as having nearly as much influence on the North Korean leaders as a possible visit from a U.S. envoy.

(Reporting by David Brunnstrom and Alex Dobuzinskis; additional reporting by Jack Kim in Seoul; Editing by Eric Beech and Robert Birsel)