An American professor arrested in North Korea as he tried to leave the country last month is being held for "committing criminal acts of hostility" aimed at overturning the hermit kingdom, the state-run KCNA news agency claimed.

Kim Sang-duk was detained at Pyongyang airport on 22 April. He is reportedly "under detention by a relevant law enforcement body which is conducting detailed investigation into his crimes."

Confirmation of Kim's arrest on Wednesday (3 May) comes amid heightened tensions between Washington and Pyongyang as fears grow over the reclusive nation conducting a sixth nuclear test, disregarding UN sanctions and pressure from the international community led by the US.

China, which accounts for about 90% of North Korean trade according to Politifact, has urged both sides to "re-establish dialogue and contact" to avoid tensions from escalating further.

Kim, who is also known as Tony Kim, is 58-years-old according to the Associated Press (AP). He is the third American citizen to be held in North Korea after Otto Warmbier, 22, and Kim Dong-chul, 62.

KCNA reported that Kim had taught an accounting course in the capital city. "Invited to Pyongyang University of Science and Technology (PUST) to teach accounting as a professor, he was intercepted for committing criminal acts of hostility aimed to overturn the DPRK [North Korea's official name] not only in the past but also during his last stay before interception."

Sweden has had diplomatic relations with North Korea since 1973 and maintains an embassy in the capital city. It provides a number of different services to foreign countries and functions as a protective power for the US and others, which includes consular responsibility for its citizens.

"We have been informed and can confirm there has been a detention of a US citizen," said Martina Aberg, deputy head of mission for the Swedish Embassy in North Korea. "He was prevented from getting on the flight out of Pyongyang. We can't comment further than this."

The university told Reuters there is no connection between Kim's arrest and his work there. An unidentified spokesman also confirmed that Kim's wife was with him when he was detained and has since returned to the US.

"Mrs Kim left the DPRK and is now back in the USA with family and friends," the spokesman was quoted as saying. "We certainly hope for a positive resolution as soon as possible."

Warmbier and Kim Dong-chul are serving 15 and 10 years hard labour respectively.