North Korea has arrested an American professor who worked at a university with strong Christian links in Pyongyang as he attempted to leave the country.

Tony Kim, also known as Kim Sang-Duk, was detained on Saturday,  according to  Park Chan-mo, chancellor of Pyongyang University of Science and Technology, also known as PUST.

It is believed that three men with US citizenship are now in custody in North Korea.

Kim, who previously taught in China, had been at Pyongyang University of Science and Technology for about a month. The university is the country's first private university and has strong Christian roots. 

The arrest comes at a time of escalating tensions between the United States and North Korea. The US has an aircraft carrier, the Carl Vinson, in the region, carrying out drills with Japanese defence forces, while North Korea is threatening another nuclear test.  China is urging restraint and US President Donald Trump tweeted before the weekend:

The original source of the news report was South Korea's Yonhap News Agency which revealed that Kim, who has joint Korean and US citizenship, had been arrested at the airport. It also said he had been involved in relief programmes to North Korea.

The university said in a statement he was detained for 'matters not connected in any way' with his work there. 

'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 statement said. 'Life on campus and the teaching at PUST is continuing as normal.' 

Pyongyang University is funded largely by evangelical Christians.

According to the BBC, founder and president Dr James Chin-Kyung Kim, a Korean-American Christian entrepreneur, was invited by the regime to build a university based on a similar one he had opened in northern China and raised much of the £20 million it cost from American and South Korean Christian charities. 

Here is a video about PUST and its founder.

North Korea heads the World Watch List of the 50 countries in the world where Christian persecution is most severe. Tens of thousands of Christians are incarcerated in horrific labour camps, and thousands more keep their faith in Christ a complete secret, according to persecution charity Open Doors.

Among foreign citizens in custody there is Canadian pastor Hyeon Soo Lim, who travelled frequently to North Korea for charity and humanitarian work and who was arrested in 2014 sentenced to hard labour for life on December 16, 2015, after at least 10 months in detention. He was charged with 'harming the dignity of the supreme leader', 'trying to use religion to destroy North Korea' and 'assisting North Koreans' escape'. He pleaded guilty to the charges.