Tony Kim, 58, also known as Kim Sang Duk, is being held in North Korea after being accused of carrying out "hostile acts with an aim to subvert the country", Korean Central News Agency (KCNA) has said.
A former accountant in California, he was detained at Pyongyang Airport as he tried to board a plane to leave the country on April 22, but officials only confirmed he had been detained and why today.
A statement from KCNA, said: ”He was intercepted for committing criminal acts of hostility aimed to overturn the DPRK not only in the past but also during his last stay before interception.”
The American citizen's detention comes as tensions between North Korea and the US are reaching an all-time high, with Mr Kim's arrest bringing the total number of Americans being held in the country to three.
Mr Kim is a Korean-American former professor at the Yabian University of Science and Technology (YUST) in the northeastern Chinese city of Yanji, near the Chinese border with North Korea.
He had just completed a one-month stint as a guest lecturer in international finance and management at the Pyongyang University of Science and Technology (PUST), a sister insistution of YUST, according to Park Chan-mo, chancellor of PUST.
He is also believed to have been working as a regional director for a humanitarian organisation for several years and had been providing aid to areas which had suffered from flooding last summer, the Korean language Voice of America Korea website said.
Mr Kim was also welll-known for visiting North Korean orphanages, providing food aid for children.
Tony Kim was detained in North Korea
The humanitarian academic is one of three US citizens held in North Korea, with Otto Frederick Warmbier arrested in January 2016 and Kim Dong Chul arrested in October 2015, although his status was not known until January 2016.
Because the US does not have diplomatic relations with North Korea, the Swedish embassy is handling consular matters involving US citizens in the country.
In reference to Tony Kim, Martina Aberg, deputy head of mission for the Swedish Embassy in North Korea, said: "We have been informed and can confirm there has been a detention of a US citizen.
"He was prevented from getting on the flight out of Pyongyang. We don't comment further than this."
Explosive images as South and North Korea test their deadly power
Tue, May 2, 2017
South Korea and US military held the integrated live fire exercise amid tensions over nuclear issues in the Korean Peninsula
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South Korean K1A2 tanks fires live rounds
On Monday the US revealed its Terminal High Altitude Area Defence (THAAD) missile defence system is now operational from a disused golf course in southern South Korea.
Last Saturday North Korea launched a ballistic missile, which is widely believed to have been its fifth failed test, and is in direct violation of UN Security Council resolutions.
North Korean leader Kim Jong-un has responded to an increasing swarm of US and Japanese war ships and aircraft by saying the country is "fully ready" for armed conflict and would not hesitate to use nuclear warfare, which he described as a "treasured sword".
Speaking through a Pyongyang-based newspaper, which acts as a state mouthpiece, he said: "The gangster-like US imperialists have gone extremely reckless in their moves to unleash a nuclear war on the Korean peninsula.
US Sea Hawks have been practicing in the Philippine Sea
“What merits a serious attention is that such military game is underway when Trump and other US warmongers are crying out for making a preemptive nuclear strike at the DPRK day after day.
“The reckless military provocation is pushing the situation on the Korean peninsula closer to the brink of nuclear war.
“The army of the DPRK is keenly watching the military movement of the US imperialists, fully ready to react to all forms of war they will opt for with a powerful nuclear treasured sword for self-defence.”
Kim Jong-un is cracking down on US citizens following US actions
At the beginning of April Donald Trump sent the USS Carl Vinson-led aircraft carrier strike group to the Korean peninsula and since then the US has increased its firepower in the region.
Last week US ally, Japan, joined in drills with the strike group, described as an "armada" by Mr Trump.
Yesterday Japan revealed its biggest warship, 294-metre long helicopter carrier the Izumo, will accompany a US supply vessel dispatched to refuel American naval forces in the region, including the USS Carl Vinson carrier group, which North Korea has threatened to sink.
Reacting to the move, an editorial in the North Korea's ruling Workers' Party newspaper, Rodong Sinmun, blamed Japan for the rising tensions and said the Land of the Rising Sun would be engulfed in radioactive clouds if nuclear war broke out.