What you need to know about the 4 Americans detained in North Korea

— -- As tensions flare between the United States and North Korea, the number of American citizens detained by the North's authoritarian government has also risen.

On Sunday, Kim Hak-song became the fourth American citizen currently detained by the repressive regime, on charges such as espionage and crimes against the state.

Kim is now the tenth American to be detained by North Korea's current leader, Kim Jong Un. In terms of detentions, North Korea is second only to Iran, where five Americans are currently missing or detained.

Although Donald Trump tweeted during the campaign that similar detentions would not happen if he became president, Kim Hak-song is the second American to be detained in the largely isolated North Korea since Trump's inauguration.

Other than their detention, there is very little that the four men have in common. Here are the American citizens detained in North Korea.

Kim Dong-chul

The American who has been held the longest in North Korea at this time is Kim Dong-chul. A naturalized American citizen who was born in South Korea, he is a businessman in his mid-60s who at one time lived in Fairfax, Virginia.

He was living in China near the border with North Korea and working in a special economic zone in North Korea as the president of a trade and hospitality company. After years of openly crossing the border, he was detained in October 2015, accused of being a spy for South Korea and sentenced to 10 years of hard labor.

He has a wife and two daughters in China. His detention wasn't publicly known until January 2016, when a CNN crew was allowed to interview him.

Otto Warmbier

Around the time that Kim Dong-chul's detention was revealed, Otto Warmbier was also arrested by North Korean authorities.

Warmbier is a 21-year-old University of Virginia undergraduate student and a native of Cincinnati. He was visiting the country on one of a handful of a guided tours organized by private companies, in which foreign tourists are chaperoned by North Korean minders and shown the country's good side.

On Jan. 2, 2016, as he was departing through Pyongyang's airport, he was detained. He was accused of stealing a propaganda poster, and North Korean officials released a video that purported to show him taking it. He was then sentenced to 15 years of hard labor for crimes against the state.

Like Kim Dong-chul, Wambier tearfully begged for mercy and read a confession, although it is unclear if it was done under duress.

Tony Kim

Tony Kim, who also goes by his Korean name, Kim Sang-duk, is a 58-year-old American citizen who was temporarily teaching an accounting course at the Pyongyang University of Science and Technology.

He was detained at the airport while trying to fly to China with his wife on April 22. It is unclear what prompted his detention, but he has been charged with hostile criminal acts with an aim to subvert the country.

Kim Hak-song

Little is known about Kim Hak-song.

The regime's official news agency said Sunday that he was being held for "hostile acts against the country," although details about the alleged acts were not provided by North Korean authorities.

Kim had also been working at the Pyongyang University of Science and Technology, in agricultural development with its "experimental farm," the university said in a statement. He was arrested Saturday as he was "about to leave … after a visit of several weeks," it added.

The university said that Kim Hak-song, like Tony Kim, was detained not because of his university work but for other reasons.