Story highlights Three months of manual labor have taken a toll on Kenneth Bae's health

He was sentenced to 15 years in a labor camp in April

His sister says he was moved to a hospital after his health deteriorated

He's suffering from leg and back pain and severe stress, she says

Kenneth Bae, the American citizen sentenced to 15 years in a North Korean labor camp , has been moved to a hospital after a serious deterioration in his health, his sister said.

Detained in North Korea in November and sentenced in April for "hostile acts to bring down its government," Bae is now suffering from severe back and leg pain and has lost more than 50 pounds, his sister Terri Chung told CNN late Sunday.

Chung said she received the information from the U.S. State Department, which told her the Swedish ambassador to North Korea had visited Bae in the hospital on Friday. Sweden represents U.S. interests in North Korea because the United States has no diplomatic presence in the secretive state.

Detention in North Korea has taken a heavy toll on Bae, who had already been dealing with other health problems, including diabetes.

"I think the last three months in the labor camp have certainly been very trying on both his mental and physical health," Chung said by phone from the Seattle area.

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'Tremendous' stress

The eight hours per day of farm labor, which Bae had never done before, appear to have worsened his symptoms, she said, adding that "he's also under a tremendous amount of stress."

Other problems he's suffering from include kidney stones, dizziness, blurred vision and loss of vision.

The family is "extremely concerned" about the situation and is pleading with the U.S. government to help Bae's case, Chung said.

U.S. officials have repeatedly called on North Korea to release Bae, whose sentence was announced toward the end of a period of heightened tensions between the two countries. The State Department called for Bae's release again in a news conference Monday.

"We continue to urge the DPRK authorities to grant Mr. Bae amnesty an immediate release," said Marie Harf, the department's deputy spokeswoman. "It's clear that Mr. Bae's health is deteriorated. We've been concerned about his health for a long time, and is of deep concern to us today.

But Kim Jong Un's regime has so far showed no sign of budging on the case of Bae, who they call Pae Jun Ho.

There didn't appear to be any recent articles published about Bae by North Korea's state-run Korean Central News Agency.

Hoping for release

In a video released early last month , Bae appealed to North Korean authorities for forgiveness and asked the United States for help in securing his release.

"Although my health is not good, I am being patient and coping well," Bae said, his head shaved and his face noticeably thinner than in earlier photos.

in that interview, he spoke of health problems including diabetes, high blood pressure, fatty liver and a back problem.

"I know what I did is not easily forgivable, but I hope that things will work out so that I can be with my family again soon," he said.

Chung had previously said that Bae is the owner of a tour company and was in North Korea for work.

North Korean state media have detailed the reasons why Bae was sentenced.

Among the list of alleged crimes was setting up bases in China for the purpose of "toppling" the North Korean government, encouraging North Korean citizens to bring down the government and conducting a "malignant smear campaign."

State media also say that Bae had planned what it called a "Jericho operation" to bring down North Korea through religious activities. They have suggested that Bae could have been sentenced to death, but avoided it through "candid confession of his crimes."