Kenneth Bae, a Korean American tour guide and evangelist imprisoned in North Korea for more than a year, has been transferred from a hospital to a labor camp, the U.S. State Department said Friday.

State Department spokeswoman Jen Psaki expressed deep concern about the development in a statement.

"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.

Bae was arrested upon entering the North Korean city of Rason in November 2012 and charged with "hostile acts against the state." He is serving a sentence of 15 months hard labor, but was moved during the summer from a prison camp to a hospital for foreigners in Pyongyang, the North Korean capital.

Officials from the Swedish Embassy, which represents U.S. interests in North Korea, have met with Bae 10 times since his detention, including Friday at a labor camp, Psaki said in the statement.

"We continue to work actively to secure Mr. Bae’s release," she said. "Per our long-standing offer, we remain prepared to send Ambassador (Robert) King to North Korea in support of Mr. Bae’s release."

King, the U.S. special envoy for North Korean human rights issues, had been expected to visit Pyongyang in October and sound out officials about prospects for an early release for Bae. But the North Korean Foreign Ministry retracted King’s invitation, saying the diplomatic atmosphere had been soured by U.S. air maneuvers over South Korea.