A U.S. citizen detained in North Korea for 15 months has been returned to a labor camp, prompting worries about his health, his sister said Friday.

Kenneth Bae, who led tour groups in North Korea, was arrested in late 2012 and sentenced to 15 years’ hard labor for unspecified hostile acts. Calls for his release on humanitarian grounds have gone unanswered. After he lost 50 pounds, he was moved last summer to a hospital from a prison work camp where he had been farming vegetables.

His sister, Terri Chung of Edmonds, Wash., said her family learned from the State Department on Friday that Bae had been taken back to the labor camp Jan. 20.

State Department spokeswoman Jen Psaki said in a statement Friday that the U.S. government was “deeply concerned” about the development.

“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,” Psaki said.

Psaki said the Swedish Embassy in Pyongyang has regularly sought and attained consular access to Bae and reiterated that the United States has a long-standing offer to send its envoy on North Korean human rights issues, Robert King, to the country.

In a report in the Choson Sinbo, a pro-Pyongyang newspaper based in Japan, Bae said he had been notified that King could visit as soon as Monday and no later than the end of the month.

Such diplomatic outreach to North Korea is unpredictable, particularly because Washington and Pyongyang do not have formal ties. The North invited King to visit in late August to seek a pardon for Bae, but withdrew the invitation at the last minute to protest U.S. military exercises. More military drills involving the United States and South Korea are planned this month.

Analysts say North Korea has previously used detained Americans as leverage in its standoff with the United States over its nuclear and missile programs, although Pyongyang said last May it would not use Bae as a bargaining chip.

Bae, 45, a father of three, was born in South Korea and immigrated to the United States with his parents and sister in 1985. He had been living in China as a Christian missionary for about seven years before his arrest. Within the past few years, he began leading small tour groups, mostly of U.S. and Canadian citizens, into a “special economic zone” designed to encourage commerce in northeastern North Korea.

Bae’s family has said he suffers from diabetes, an enlarged heart, liver problems and back pain. When his mother was allowed to visit him last October, he could stand for only 30 to 45 minutes at a time, his sister said.

— Associated Press

Matthew Pennington contributed to this report.