North Korea has detained its fourth US citizen as part of Kim Jong Un's strategy to use American hostages as bargaining chips.

Kim Hak Song was detained in the dictatorship 'on suspicion of acts against the state' on May 6, according to the state news agency.

Song worked for the Pyongyang University of Science and Technology alongside another US citizen, Kim Sang-duk, who was detained for 'hostile acts' last month - making him the third American to be detained in North Korea.

The other two Americans already held in the isolated country are Otto Warmbier, a 22-year-old student and Kim Dong Chul, a 62-year-old Korean-American missionary.

North Korea has detained its fourth US citizen, Kim Hak Song, who worked for the Pyongyang University of Science and Technology alongside another US citizen Tony Kim, 58, (left and right) who was detained last month

The other two Americans already held in the isolated country are Otto Warmbier, a 22-year-old student, (right) and Kim Dong Chul, (left) a 62-year-old Korean-American missionary.

Warmbier was detained in January 2016 and sentenced to 15 years hard labour for attempting to steal a propaganda banner. Two months later, Kim Dong Chul was sentenced to 10 years hard labour for subversion. Neither has appeared in public since their sentencing.

Now Song makes four US captives in the isolated country.

North Korea has in the past used detained Americans to extract high-profile visits from the United States, with which it has no formal diplomatic relations.

Indeed, former president Bill Clinton traveled to North Korea in 2009 and met with officials. Un's father, Kim Jong-il, went on to pardon American journalists Euna Lee and Lisa Ling, who had been sentenced to 12 years of hard labor.

But his son appears less willing to negotiate.

North Korean leader Kim Jong Un is believed to be taking American hostages to use as bargaining chips

North Korean leader Kim Jong-Un (center) hopes the detainees will act as human shields against American attack, experts say

In June, North Korean state media said the country will not negotiate with the US over the Americans citizens it had captive, until former detainee Kenneth Bae stops publicly talking about his time in prison.

North Korea arrested Bae, a US missionary, in November 2012 and sentenced him to 15 years' hard labour for crimes against the state. He was released two years later, along with Jeffrey Edward Fowle and Matthew Todd Miller after James Clapper, then the director of national intelligence, made a secret visit to Pyongyang.

Bae has written an account of his detention in a memoir and spoken about his experiences at several public appearances - to the fury of Kim Jong Un and North Korea officials.

Experts believe that Kim Jong Un is holding Americans, not to get a seat at the negotiating table, but to act as human shields amid fears the US may attack amid worsening tensions between North Korea and the United States.

'Kim Jong Un is using hostage diplomacy as a part of his military and defense strategy with focus on preventing the U.S. from removing him from power as well as to prevent the U.S. from taking military options against North Korea, ' Dr. An Chan Il, president of the World Institute for North Korea Studies and a former defector, told NBC News.

Both Kim Hak Song and Kim Sang-duk, were working at the Pyongyang University of Science and Technology (pictured) before being detained

The latest detention will likely be seen as an act of aggression by President Trump who has warned he will consider military action against North Korea.

But Dr Koh Yu Hwan, a professor of North Korean studies at Dongguk University in Seoul, said that even if the step does not lead to negotiation, it could limit the options faced by the US for an attack.

'This is North Korea's hostage diplomacy,' Dr. Koh said. 'They are like the human shields against the U.S. government with the ultimate goal of attention grabbing.'

Detaining American hostages for 'crimes against the state' also allowed Kim Jong Un to feed the propaganda rhetoric to his citizens than North Korea was constantly under attack from outside forces.

Dr An believes North Korea will detain more Americans if they can.

'North Korea considers detaining an American a measurably successful move,' she said.

Kim Dong Chul, center, a U.S. citizen detained in North Korea, is escorted to his trial last April where he was sentenced to 10 years for espionage

Pyongyang has issued increasingly belligerent rhetoric in a tense stand off with the Trump administration over its rogue weapons program.

The war of words between the West and the reclusive regime has spiked in recent weeks, and Pyongyang has threatened to carry out a sixth nuclear test that would further inflame tensions.

Trump warned in an interview with Reuters that a 'major, major conflict' with North Korea was possible, while China said last week the situation on the Korean peninsula could escalate or slip out of control.

The escalation of aggressions came after North Korea conducted a weapons test fire last month.

In a show of force, the United States sent an aircraft carrier strike group, led by the USS Carl Vinson, to waters off the Korean peninsula to conduct drills with South Korea and Japan.

Then last week, Kim Jong-un accused the US of pushing the Korean Peninsula to the brink of nuclear war after a pair of American bombers carried out training drills in the region.

The supersonic B-1B Lancers were deployed in a joint training exercise with the Japanese air forces amid heightened tensions between Pyongyang and Washington.

Warmbier told a North Korean court he tried to steal the banner as a trophy for an acquaintance who wanted to hang it in her church (March 2016 photo)

Otto Frederick Warmbier, 21 (pictured), was detained at Pyongyang airport in January 2016 before he was due to board a flight back to China after a five-day trip

North Korea said the bombers conducted 'a nuclear bomb dropping drill against major objects' in its territory at a time when Trump and 'other U.S. warmongers are crying out for making a preemptive nuclear strike'.

And on Friday, Kim Jong Un accused the CIA of plotting with South Korea to assassinate him.

Secretary of State Rex Tillerson had said previously that the U.S. was not interested in 'regime change.'

Kim Hak Song and Tony Kim, 58, who worked in California and also goes by his Korean name Kim Sang-Duk, were both working at the Pyongyang University of Science and Technology before they were arrested.

Kim Hak-song had previously described himself as a Christian missionary who intended to start an experimental farm at PUST, Reuters news agency reported.

Kim Sang-duk, who was teaching accountancy, was detained at Pyongyang Airport as he was preparing to leave the country in April and accused of trying to overthrow theNorth Korean regime.

A statement released by the Korean Central News Agency said Kim had been charged with 'committing criminal acts of hostility aimed to overturn North Korea.'

The Pyongyang University of Science and Technology is the only privately funded university in North Korea. It held its first classes in 2010. It is unique in the North for its large number of foreign staff.

President Trump (right, on Thursday) warned that he was prepared to take military action against North Korea although Secretary of State Rex Tillerson (left) said they weren't looking for a regime change

The university was founded by evangelical Christians and opened in 2010. Its students are generally children of the country's elite.

The volunteer faculty of PUST, many of whom are evangelical Christians, has a curriculum that includes subjects once considered taboo in North Korea, such as capitalism. The college is an unlikely fit in a country that has been condemned by the U.S. State Department for cracking down on freedom of religion.

No further details were available about the circumstances related to the arrests of the two men associated with the college. A spokesman for the Pyongyang University of Science and Technology was not immediately available for comment.

Warmbier, then a 21-year-old University of Virginia student from suburban Cincinnati, was sentenced to 15 years of hard labor last year after he confessed to trying to steal a propaganda banner.

Former New Mexico Gov. Bill Richardson, who has been working to get Warmbier released, said progress had been 'very slow'.

'It's not a government-to-government deal, because we have a very bad relationship with North Korea,' he said at the end of last year.

Richardson told Fox News, 'in the past, we could talk to (Kim Jong Un's) father [Kim Jong-il] when he was the president and you could make deals, but now there's just like silence. There's nothing coming back.'

Kim Dong Chul, who was born in South Korea but is also believed to have U.S. citizenship, is serving a sentence of 10 years for espionage.

Another foreigner, a Canadian pastor, is also being detained in North Korea.

Hyeon Soo Lim, a South Korean-born Canadian citizen in his 60s, was convicted and sentenced to life in prison in 2015 on charges of trying to use religion to destroy the regime and helping U.S. and South Korean authorities abduct North Korean.