The killing of Kim Jong-Nam - the estranged half-brother of North Korean leader Kim Jong-Un - in February sparked a diplomatic row which saw both countries banning each other's citizens from leaving and withdrawing their ambassadors.
The new detentions come amidst growing tensions between the US and North Korea. The nation's state media agency, KNCA, condemned President Donald Trump's deployment of a naval task force to the region, then launched into a 2,000-word editorial that accused the USA of provocative military actions and threatened not only a "preemptive nuclear strike" but promised to reduce the White House "to ashes" and ensure that America's imperialism "will be over".
"There may come a day if North Korea gives up its nuclear weapons program and so on and so forth, and having North Korea back in the community of states or in the community of states would be worthwhile", she said.
Last week, President Donald Trump called Kim a "pretty smart cookie" and said he'd be honored to meet with him.
The U.S. and its allies in the eastern Asian region are common targets of Kim Jong-Un's regime and its threats of nuclear war.
"But I don't think that you can erode the scaffolding of democracy in the United States", she added during a tour to promote her book "Democracy: Stories from the Long Road to Freedom".
North Korea has reportedly arrested a US citizen and professor from the Pyongyang University of Science and Technology who has identified himself as a Christian.
The arrest on Saturday of Kim Hak-song raises the number of US citizens now held in North Korea to four. Kim had been very concerned about the food shortage in North Korea.
While the leadership under President Xi Jinping is increasingly irked by a series of North Korea's banned weapons tests in defiance of global warnings, Beijing has said countless times that it is against the use of military force and called for dialogue to ease tensions on the peninsula.
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- American Sandra Suh was deported in April 2015 after being accused of making anti-North Korea propaganda while visiting the country. "Therefore, we can not comment on anything that Mr Haksong Kim is alleged to have done", a university statement said.
Since the USA and North Korea do not hqve diplomatic relations, when an American citizen is reported to be detained, the US works with the Swedish embassy in Pyongyang.
Korean-American Tony Kim, who also goes by his Korean name Kim Sang-duk and is in his 50s, was the first to be arrested last month from PUST.
- Americans Matthew Miller and Kenneth Bae went home in November 2014 after their release was secured through a secret mission by the top USA intelligence official.
Another American citizen has been taken hostage by North Korea.
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Kim worked at the Pyongyang University of Science and Technology.
In an emailed statement, a State Department official said: "The security of US citizens is one of the department's highest priorities".
And although President Trump tweeted during the campaign that similar detentions would not happen during his presidency, Kim Hak Song is the second American to be detained in North Korea during his time in office.
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