Three Americans are hostages in North Korean prisons amid nuclear stand-off

A US citizen working as a university lecturer has been detained by authorities in North Korea – becoming the third American to be held in the country.

Korean-American Kim Sang-duk, also known as Tony Kim, taught at Pyongyang University of Science Technology (PUST) for a month before being arrested on Saturday (22 April) while trying to leave the country.

The reason for his detention is not known but comes as tensions ratchet up over North Korea's nuclear weapons and missile testing.

In the latest warnings, Pyongyang has threatened to carry out a nuclear strike on Australia and sink the US aircraft carrier USS Carl Vinson, which is beginning joint drills with two Japanese destroyers in the western Pacific Ocean.

At least two other US citizens are currently being detained in North Korea, with the country's ruling regime in the past accused of detaining Americans to use as bargaining chips with the US, with which it has no formal diplomatic relations.

Otto Warmbier, then a 21-year-old University of Virginia student from Cincinnati, was last year sentenced to 15 years of hard labour after confessing to the attempted theft of a propaganda banner. Kim Dong-chul, born in South Korea but who says he has US citizenship, is serving a 10-year sentence for espionage after confessing to trying to steal military secrets for South Korea.

It is not known if the admissions of guilt were coerced, with other US citizens detained in North Korea having recanted their confessions following their release.

IBTimes UK takes a look at each of the three US citizens currently being held in North Korea.

Kim Sang-duk (also known as Tony Kim)

Korean-American Tony Kim, also known as Kim Sang-duk, was reportedly arrested by North Korean authorities on Saturday at Pyongyang International Airport while trying to travel to China with his wife.

Kim, 58, had spent a month teaching an accounting course at the Pyongyang University of Science and Technology (PUST), the university's chancellor, Chan-Mo Park, told Reuters.

"The cause of his arrest is not known but some officials at PUST told me his arrest was not related to his work at PUST. He had been involved with some other activities outside PUST such as helping an orphanage," Park said. "I sincerely hope and pray that he will be released soon".

Kim is listed as an accounting professor on the website of Yanbian University of Science and Technology (YUST) in China, which is affiliated to PUST. He was also said to be involved in aid programmes for children in rural parts of North Korea, South Korean news agency Yonhap said.

His detention was confirmed by the Swedish embassy in North Korea, which looks after consular affairs for the US.

Otto Warmbier

Last year, Otto Warmbier, then a 21-year-old University of Virginia student from Cincinnati, was sentenced to 15 years of hard labour after confessing to trying to steal a propaganda banner.

He had been on a guided tour with a travel company but was detained on 2 January 2016 at Pyongyang airport while trying to leave the country. In announcing his arrest, the state news media said Warmbier had visited North Korea with the intent of "bringing down the foundation of its single-minded unity".

He was alleged to have been encouraged by the CIA, a secretive American university organisation and a member of a church in Ohio to steal a political poster from a wall in his hotel. The poster said, "Let's arm ourselves strongly with Kim Jong-il's patriotism!"

Footage broadcast on state television after his arrest showed Warmbier with his head down and shackled as he tearfully apologised to officials and pleaded for his release.

He was convicted in March 2016 of subversion after a one-hour trial at North Korea's Supreme Court. He had confessed to stealing the propaganda poster as a "trophy" for the mother of a friend who wanted to hang it in their church in Wyoming, Ohio. In exchange, he would receive a used car worth $10,000 (£7,800, €9210).

He said stealing the propaganda banner from the staff-only area of a Pyongyang hotel was "the worst mistake of my life".

The US State Department has repeatedly called for Warmbier's release, accusing North Korea of using US citizens as "pawns to pursue a political agenda".

Phil Robertson, the deputy Asia director for Human Rights Watch, said at the time that the punishment of "15 years' hard labour for a college-style prank is outrageous and shocking".

Kim Dong-chul

Kim Dong-chul, born in South Korea but said to hold US citizenship, is serving a 10-year sentence for espionage. He said he was arrested in October 2015 after officials accused him of spying for the South Korea government.

In an interview with CNN in January last year, Kim said he was a 62-year-old naturalised US citizen from Fairfax, Virginia. The married father-of-two said he used to run a trading and hotel services company in Rason, a special economic zone that North Korea operates near its borders with China and Russia.

He said he started working as a spy in April 2013, bribing North Korea residents to "gather important materials".

"I was tasked with taking photos of military secrets and 'scandalous' scenes," he also told CNN. He said he was arrested while trying to extract classified information from a former North Korean soldier. It could not be certain if the comments were made under duress.

Pyongyang said when Kim was arrested in Rason he had in his possession a USB stick containing military and nuclear secrets.

He was accused of carrying out "reactionary propaganda" against North Korea "and injected into local people fantasies about the superiority of the United States, in order to shake the stability of the political and social system", China's official Xinhua news agency reported.

In March last year, Kim appeared at a government-arranged news conference where he confessed to trying to steal military secrets in collusion with South Korean agents. The South Korean spy agency has denied any involvement in such a plan.

According to Chinese state news agency Xinhua, Kim said he was first introduced to South Korean spies by US intelligence officers.

Kim was sentenced to 10 years of hard labour in April 2016.