North Korea has confirmed the arrest of a US citizen who was lecturing in Pyongyang, accusing him of "acts of hostility".

Kim Sang-duk, also known as Tony Kim, was arrested at the capital's airport on April 22 as he tried to leave the country after teaching for several weeks at the Pyongyang University of Science and Technology (PUST).

He is the third US citizen being held in the country.

In North Korea's first confirmation of the professor's detention, the official Korean Central News Agency (KCNA) said he had been held for "committing criminal acts of hostility aimed to overturn the DPRK", using the acronym for the Democratic People's Republic of Korea.

It added that Kim was "under detention by a relevant law enforcement body which is conducting detailed investigation into his crimes".

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Last month, PUST officials said the professor was being held, but did not comment further on his case.

"We cannot comment on anything that Mr. Kim may be alleged to have done that is not related to his teaching work on the PUST campus," the university, which was founded by evangelical Christians in 2010 and has a number of American faculty members, said in a statement.

According to US media, the state department is aware of Kim's detention and is working with the Swedish Embassy on his case.

The embassy looks after consular affairs for the US in North Korea because the two countries do not have diplomatic relations.

The confirmation of Kim's detention comes as tensions remain high between Washington and Pyongyang.

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North Korea said on Monday it was prepared to carry out a nuclear test "at any time and at any location" after US officials said the US military's missile defence system in South Korea reached an initial operating capability to defend against the North's missiles.

US President Donald Trump also said this week he would be "honoured" to meet North Korean leader Kim Jong-un under the right conditions.

The White House promptly told the press that conditions were not right yet.

At least two other US citizens are currently being held in the North after being sentenced to long prison terms.

Last year, Otto Warmbier , then a 21-year-old University of Virginia student, was sentenced to 15 years of hard labour in prison after he confessed to trying to steal a propaganda banner.

Kim Dong-chul, who was born in South Korea but is also believed to have US citizenship, is serving a 10-year sentence for espionage.