Story highlights Kenneth Bae was arrested in North Korea in November

North Korea's supreme court sentenced him to 15 years of hard labor

Bae's sister says his health has seriously deteriorated

A State Department special envoy will travel to North Korea this week to try to free Kenneth Bae, the U.S. citizen detained there since November, the State Department and White House said Tuesday.

Ambassador Robert King, the president's special envoy for North Korean human rights issues, will head to Pyongyang at North Korea's invitation, the State Department said.

King, currently traveling in the region, will go to the capital Friday, the White House said.

North Korea's supreme court sentenced Bae in April to 15 years of hard labor. His sister, Terri Chung, told CNN two weeks ago that Bae was recently moved to a hospital because of a serious decline in his health.

The court found Bae guilty of carrying out "serious crimes" against North Korea, including 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 smear campaign, according to the country's state media.

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The media also say Bae planned an operation to bring down the government through religious activities.

Chung says her brother was the owner of a tour company who was in North Korea for work.

King will ask Pyongyang to pardon Bae and grant him special amnesty on humanitarian grounds "so that he can be reunited with his family and seek medical treatment," the State Department said.

Bae suffers from severe back and leg pain and has lost more than 50 pounds, Chung told CNN earlier this month.

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

Bae also suffers from kidney stones, dizziness, blurred vision and loss of vision, Chung said. He was already dealing with other health problems, including diabetes.