South Korea's unification ministry said Monday it is checking information about North Korea's latest detainment of a US citizen amid tensions over Pyongyang's nuclear and missile programs.

Tony Kim, a Korean-American man in his late 50s, was arrested Saturday at an airport in Pyongyang, becoming the third US citizen held in captivity in the communist country, according to sources.

"The government is not aware of Kim's whereabouts after his visit to North Korea, as he is not a South Korean national," Lee Duk-haeng, ministry spokesman, told a press briefing. "We are checking the relevant information through multiple channels."

The man, who also goes by his Korean name Sang-duk Kim, was arrested when he was about to leave the country "after several weeks of service teaching at Pyongyang University of Science and Technology," the university said.

"We understand that this detainment is related to an investigation into matters that are not connected in any way with the work of PUST," the university in North Korea said in a statement.

"We cannot comment on anything that Mr. Kim may be alleged to have done that is not related to his teaching work and not on the PUST campus," it added.

He is known to be a former professor at the Yanbian University of Science and Technology in China and to have been engaged in aid and relief programs to North Korea.

A source familiar with North Korean affairs said that Kim was involved in a project to help North Korean children in mountainous areas of the country's northeastern provinces bordering China.

"He would not have conducted any anti-North Korea activities," said the source.

North Korea's state media has yet to release reports on his detention.

His case marked the third detention of an American national by North Korean authorities. Pyongyang is holding two American citizens -- Kim Dong-chul, a 62-year-old Korean-American and Otto Warmbier, a US college student.

Last year, both were sentenced to multiple years of hard labor for what Pyongyang calls subversive acts against North Korea.

In July 2016, the North said it will handle Americans detained in the country with martial law, without disclosing details.

Lim Hyeon-soo, a Korean-Canadian pastor, has also been detained in the North since he entered the country via China on a humanitarian mission in January 2015.

Experts said the North has previously used detained Americans as leverage to force the US to open negotiations with it.

In July 2016, the North said it will handle Americans detained in the country with martial law, without disclosing details.

Lim Hyeon-soo, a Korean-Canadian pastor, has also been detained in the North since he entered the country via China on a humanitarian mission in January 2015.

In 2014, Pyongyang released three detained Americans -- Kenneth Bae, Matthew Todd Miller and Jeffrey Fowle. (Yonhap)