State department officials and Kenneth Bae’s sister were quoted as saying the 45-year-old had been returned from a hospital to the camp on January 20.
Kenneth Bae, a Korean-American, was arrested in November 2012 and sentenced to 15 years’ hard labor in May.
North Korea says Kenneth Bae used his tourism business to form groups to overthrow the government.
He was taken to hospital last year after suffering dramatic weight loss. His family says he has several health complaints including diabetes and liver problems.
State Department spokeswoman Jennifer Psaki said Washington had learned about Kenneth Bae’s transfer to the camp from representatives of the Swedish embassy in Pyongyang, which acts on behalf of the US in North Korea.
Jennifer Psaki said the Swedish diplomats “have met Mr. Bae 10 times since his detention, most recently on February 7 in a labor camp”.
She added: “We continue to urge DPRK (North Korean) authorities to grant Mr. Bae special amnesty and immediate release on humanitarian grounds.”
Jennifer Psaki did not specify when Kenneth Bae had been forced back to the camp.
However, a US state department official and Kenneth Bae’s sister confirmed the January 20 date.
“He’s back to eight-hours-a-day, six-days-a-week hard labor,” Terri Chung, Kenneth Bae’s sister, was quoted as saying by the Associated Press news agency.
Pyongyang has so far made no official comment on the reports.
On Thursday, President Barack Obama used the annual National Prayer Breakfast in Washington to say: “We pray for Kenneth Bae, a Christian missionary who has been left in North Korea for 15 months.”
“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.”
If confirmed, Kenneth Bay was returned to the camp on the same day as spoke to foreign media in North Korea under heavy prison guard – his first “press conference” since the detention.
Kenneth Bae denied media reports that he had been badly treated and called for US “co-operation” to secure his release.
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