An American missionary detained in one North Korea's labour camps has been taken to hospital, his family has said.

Kenneth Bae, a 45-year-old tour operator and Christian missionary, was arrested in November and sentenced to 15 years' hard labour for subversive activities.

He has lost more than 50lbs (22kg), his sister, Terri Chung, who lives near Seattle, said.

Bae suffers from diabetes, an enlarged heart, liver problems and back pain. Chung said that a deputy ambassador from Sweden met Bae in hospital on behalf of the US as Washington has no diplomatic relations with the Pyongyang regime.

"He's considerably weaker," Chung said. "There's more urgency than ever to bring him home.

"The last three months in the labour camp have certainly been very trying on both his mental and physical health."

In letters to his family, Bae described how he had been forced to work on farms, weeding and planting beans and potatoes.

The US State Department has called for his release on humanitarian grounds.

Bae was born in South Korea but is a naturalised American citizen. Pyongyang accused him of plotting to topple the North Korean government through a "malignant smear campaign" and his religious activities.

South Korean rights groups have suggested that authorities were upset because Bae, a devout Christian, repeatedly took pictures of starving children in the streets and of public executions of dissenters.

Journalists Laura Ling and Euna Lee were sentenced to 12 years' hard labour in 2009 after illegally crossing the border from China while making a documentary about defectors. They were freed after Bill Clinton flew to Pyongyang to negotiate their release.