His arrest and conviction came as the DPRK and the US remain locked in a diplomatic standoff surrounding Pyongyang's missile and nuclear tests and its claim that Washington was plotting to attack the country.

In two months of daily verbal assault earlier this year prompted by annual drills by militaries from the US and the Republic of Korea, Pyongyang threatened to attack the two allies.

The DPRK has had a history of trying to use US captives as a bargaining chip to persuade Washington to hold talks, but the administration of US President Barack Obama has been reluctant to respond.

Bae has sent letters to his family in the US pleading for help because his health was failing, his sister said in a media interview last week.

Carter has made prior trips to Pyongyang on diplomatic missions. In 2010, he helped earn the release of another US citizen, Aijalon Mahli Gomes, a Boston native who was sentenced to eight years hard labor for illegally entering the country.

Former US president Bill Clinton flew to the DPRK in 2009 and won the release of two US women media workers who were sentenced to 12 years for illegally entering the country.