North Korea has moved imprisoned US citizen Kenneth Bae from hospital and back to a labour camp, a US official said, which could further complicate efforts to win his release.

Bae, a tour operator described by a North Korean court as a militant Christian evangelist, was arrested in November 2012 and later sentenced to 15 years' hard labour on charges of seeking to topple the government.

His family says that the 45-year-old is seriously ill.

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

She said Washington was "gravely concerned" about the health of the Korean American, who has been in detention for more than 15 months.

It was not immediately clear when Bae was moved to the labour camp, but Psaki said Swedish embassy officials visited him there on Friday.

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

'Anti-government acts'

Bae, who is also known as Pae Jun-Ho, was detained as he entered North Korea's northeastern port city of Rason.

He began serving his jail sentence in a prison camp in May 2013. But in August he was admitted to hospital after he lost more than 23 kilos and began having kidney and liver problems.

On Thursday, US President Barack Obama renewed a call for Bae's release at the annual national prayer breakfast, saying "let us never forget those who are persecuted today, among them Americans of faith".

"His family wants him home, and the United States will continue to do everything in our power to secure his release because Kenneth Bae deserves to be free," Obama said.

Last month, Secretary of State John Kerry met Bae's relatives at the State Department, in an apparent signal to Pyongyang to release him.

Bae apologised in January in a brief appearance before reporters in Pyongyang, acknowledging he had participated in anti-government acts - a public confession that observers saw as a pre-requisite for any release.

His family, which has campaigned hard for his freedom, also apologised in a statement.

The State Department contacted Bae's family on Friday "as soon as we received the Swedish consular report," Psaki said.

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.

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

"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.

King was invited to North Korea to discuss Bae's case at the end of August, but Pyongyang rescinded the invitation at the 11th hour without explanation.