The diplomatic source was quoted as saying that the issue "is becoming a burden for the United States.

　　"Even if Carter's visit materializes, it will be focused on the issue of Kenneth Bae's release more than anything else."

　　The DPRK's Supreme Court sentenced Bae in May to 15 years hard labor after he was detained in November leading a tour group through the northern region of the country.

　　Pyongyang said Bae was participating in activities designed to overthrow the government by infiltrating at least 250 students into the country.

　　Bae has acknowledged to being a missionary and has said he had conducted services in the DPRK.

　　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.