WASHINGTON (AFP) - North Korea has moved imprisoned United States (US) citizen Kenneth Bae from a hospital to a labour camp, a US official said on Friday, voicing fears for his health.

"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," said spokeswoman Jen Psaki, adding Washington was "gravely concerned about his health".

It was not immediately clear when the 45-year-old Korean American was moved to the camp, but Ms Psaki said Swedish embassy officials visited him there on Friday.

"We continue to urge DPRK (North Korea) authorities to grant Mr Bae special amnesty and immediate release on humanitarian grounds," she added.

Mr Bae, a tour operator described by a North Korean court as a militant Christian evangelist, was arrested in November 2012.

He was sentenced to 15 years' hard labor on charges of seeking to topple the government.

As Washington does not have diplomatic ties with Pyongyang, the Swedish embassy acts on its behalf in any communications with the reclusive North Korean authorities.

Ms Psaki said in her statement that Swedish embassy officials had met 10 times with Mr Bae since his detention.

The State Department was in contact with Mr Bae's family on Friday "as soon as we received the Swedish consular report".

"We continue to work actively to secure Mr Bae's release," she said, adding that an offer was still open to send Ambassador Robert King, special envoy to North Korea, to the country in support of Mr Bae's release.