Former US president Jimmy Carter has no plans to visit North Korea, his spokeswoman said Thursday, after a report that the Nobel laureate intended to go with hopes of freeing a detained American.

"President Carter has not had an invitation to visit North Korea and has no plans to visit," spokeswoman Deanna Congileo said.

South Korea's Yonhap news agency, quoting unnamed diplomatic sources, reported Wednesday that Carter sent a letter to US Secretary of State John Kerry voicing willingness to travel to the isolated country amid tensions.

The news agency said Carter would try to free a detained Korean-American tour operator but that South Korean and US officials worried that Pyongyang could use the trip "for propaganda purposes."

North Korea earlier sentenced Pae Jun-Ho, known in the United States as Kenneth Bae, to 15 years of hard labor. He was arrested in November and accused of "hostile acts" against the totalitarian regime.

North Korea in the past has freed Americans after high-level emissaries. In 2010, Carter negotiated the release of another American, Aijalon Mahli Gomes, who was sentenced to eight years of hard labor for illegally entering the country from China.

Carter paid a landmark visit to North Korea in 1994 and negotiated an end a crisis that raised fears of war. Under an agreement, the United States and its allies agreed to assist the regime in developing civilian power.

Tensions have again soared in recent months, with North Korea under young leader Kim Jong-Un threatening nuclear war against the United States after international condemnation of a rocket launch and nuclear test by Pyongyang.