WASHINGTON (REUTERS) - The United States (US) will send its North Korean human rights envoy to Pyongyang this week to seek the release of an imprisoned US Christian missionary, the State Department said on Tuesday.

Mr Robert King, special envoy for North Korean human rights issues, will travel to Pyongyang on Friday at the invitation of the government on a "humanitarian mission focused on securing the release of US citizen Kenneth Bae," the department said in a statement.

Mr King will ask North Korea to "pardon Mr Bae and grant him special amnesty on humanitarian grounds so that he can be reunited with his family and seek medical treatment," it added.

Bae was sentenced in April to 15 years of hard labour after North Korea's Supreme Court convicted him of state subversion.

The court said Bae, 45, used his tourism business to form groups to overthrow the government.

Bae was detained in November as he led a tour group through the northern region of the country. His sentencing came amid acrimonious relations between Pyongyang and Washington over the reclusive state's nuclear program.

Bae's sister, Ms Terri Chung, told Reuters on Aug 10 that her brother had been held at a prison for foreigners and put to work plowing and planting fields.

He was transferred to a state hospital because he suffered from a range of health problems, including an enlarged heart, chronic diabetes as well as back and leg pain, she said.