Kim, whose Korean name is Kim Sang-duk, was teaching at Pyongyang University of Science and Technology, international media said, quoting a school statement. The arrest took place on Saturday. Kim reportedly was teaching accounting for about a month at the school. He also reportedly worked on humanitarian projects for the North over the past 10 years. Kim's arrest raised the number of U.S. citizens in North Korean custody to three. The detention also adds fuel to weeks of speculation that the North will soon make its sixth nuclear test. The North has a history of arresting an American before a nuclear test, handing down a prison term and releasing the detainee after some negotiations with the United States. The pattern started with the second nuclear test in 2009. The North arrested Laura Ling and Euna Lee, two TV journalists, at the Chinese border on March 17, 2009. They were convicted and sentenced to 12 years of hard labor for hostility against the North Korean people and immigration law violations. The second nuclear test was May 25, 2009. Washington began negotiation with Pyongyang and former U.S. President Bill Clinton visited North Korea on Aug. 4, 2009 and brought the two women home the next day. Before its third nuclear test on Feb. 12, 2013, Kenneth Bae, a Korean-American who operated a travel agency in China, became a victim. He was arrested on Nov. 3, 2012 and sentenced to 15 years of hard labor for hostility against the country. He was released on Nov. 8, 2014. The North also detained two U.S. citizens before its fourth nuclear test on Jan. 6, 2016. They are still in the regime's custody. Reverend Kim Dong-chul, a naturalized U.S. citizen, was arrested in October 2015 and sentenced to 10 years of hard labor. Otto Warmbier, a student at the University of Virginia, was detained on Jan. 2, 2016 after visiting the country with a tour group. He was sentenced to 15 years of hard labor for allegedly removing a political banner from a hotel wall.

 The North and the United States are negotiating their releases through the Swedish Embassy in Pyongyang and the so-called New York channel of communication at the United Nations. “The North doesn't care about an individual,” said a South Korean government official. “But it knows how much other countries, including the United States, value every individual citizen.” He pointed out that nine Malaysian diplomats and their families in Pyongyang were taken hostage after the assassination of Kim Jong-nam, the half brother of North Korean ruler Kim Jong-un, in Kuala Lumpur last February. The corpse of Kim and two North Koreans suspects in the murder were sent to Pyongyang after a diplomatic deal to release the hostages. “The North probably wants to minimize the sanctions to come from its upcoming nuclear test and hold higher ground in negotiations with the United States after the test,” the official said.