SEOUL, South Korea — North Korea said Sunday that it would not use an imprisoned American citizen as a political bargaining chip, rejecting the possibility of allowing any prominent Americans to visit the North to request his release, as it has in similar cases.

The United States has called for the immediate release of the prisoner, Kenneth Bae, a Korean-American who was sentenced last week to 15 years of hard labor for committing “hostile acts” against the North.

Mr. Bae, 44, was arrested in Rason, in the northeast, in November after leading a group of businessmen there from Yanji, China. The North’s refusal to release him adds a new source of tension in the relationship between the United States and North Korea as Washington tries to hold a tough line with the North over its nuclear program.

Mr. Bae is the sixth American known to have been arrested in the North since 2009. The rest have been released or deported. Two were released in 2009 only after former President Bill Clinton visited Pyongyang, the North’s capital, and met with Kim Jong-il, the North’s leader at the time. Another was released in 2010 after a visit by former President Jimmy Carter.