North Korea said Saturday that a detained American allegedly tried to “topple” its government and will soon be put on trial, a potential complication as Washington tries to ramp down tensions stemming from Pyongyang’s recent weapons tests and threats of nuclear attack.

Kenneth Bae, a tour operator from Washington state, is the sixth American detained by the North since 2009, but he faces more serious charges than the others. The North used several previous cases as bargaining chips with the United States, drawing rescue-mission visits from former presidents Bill Clinton and Jimmy Carter.

The North gave no specifics about its evidence against Bae, saying only that his crimes were “clearly substantiated.” In November, Bae traveled in a small group to the North Korean port city of Rajin, a special economic zone that caters to foreign investors. There, he was arrested by authorities.

Bill Richardson, a former U.S. ambassador to the United Nations, tried to negotiate Bae’s release during a trip to Pyongyang in January. But North Korean officials would not let Richardson meet with Bae, Richardson said — although they did reassure him that Bae’s health was good. They also told Richardson that Bae had been allowed to meet with members of the Swedish Embassy in Pyongyang, which occasionally acts on behalf of the United States because Washington does not have diplomatic ties with the North.

In an interview with CNN after his trip, Richardson said Bae was being held “very far away from Pyongyang, in the northern part of the country.”

Since Richardson’s visit, tensions between North Korea and the United States have spiked. In February, the North detonated an underground nuclear weapon and then responded with fury to a new round of U.N. sanctions. It also threatened preemptive nuclear strikes against the United States and its allies in the region, including South Korea and Japan.

Washington has proposed talks with the North — but only if its leaders show signs that they want to denuclearize, something they have repeatedly said they will never do.