WASHINGTON, 2013 (AFP) - The United States on Monday urged North Korea to free a jailed US citizen, warning that his health was deteriorating.

The State Department confirmed accounts by Kenneth Bae's sister that the Korean-American tour operator was moved from prison to a hospital in North Korea.

"It's clear that Mr. Bae's health is deteriorating," State Department spokesman Marie Harf told reporters.

"We will continue to urge the DPRK to grant him amnesty and immediate release so he can return home," she said, using the North's official name of the Democratic People's Republic of Korea.

The ambassador of Sweden, which takes care of US interests in North Korea in the absence of relations between Washington and Pyongyang, visited Bae in a hospital on Friday, Harf said.

It was the first such visit since May, when the ambassador saw Bae in prison, Ms Harf said.

Bae, whose Korean name is Pae Jun Ho, was arrested in November as he entered the north-eastern port city of Rason.

North Korea sentenced him to 15 years of hard labour on charges of trying to topple Kim Jong Un's regime.

The hardline communist state, which strictly bans religious proselytisation, said that Bae was a Christian evangelist who brought in "inflammatory" material.

Bae's sister Terri Chung said in weekend media interviews that her brother has lost more than 23kg and has problems with his kidneys and liver.

North Korea has in the past freed detained Americans after visits from high-level emissaries including former presidents Bill Clinton and Jimmy Carter.

But Chosun Sinbo, a newspaper that usually speaks for North Korea, said last month that Pyongyang would only release Bae if the United States changed its policy.

Bae was arrested at a time of high tension between the United States and North Korea, which in February defiantly carried out its third nuclear test.