By James Pearson and Ju-min Park

SEOUL (Reuters) - Like many other Americans who came to teach at the foreign-funded Pyongyang University of Science and Technology (PUST), Kim Hak Song was a Christian missionary who raised money from a church to come to North Korea.

Kim had been running PUST's experimental farm before he was detained on Saturday, traveling by train from Pyongyang to China's border town of Dandong, PUST's chancellor and co-founder Chan-mo Park told Reuters.

The university, which is open about its Christian affiliation, says its sole mission is to help North Korea's future elite learn the skills to modernize the isolated country and engage with the outside world. Former teachers say the faculty is careful to avoid anything that looks like missionary work.

The university attracts a steady stream of devout American Christians, despite North Korea's history of handing down long sentences with hard labor to missionaries accused of various transgressions.

North Korea has in the past used detainees to extract concessions, including high-profile visits from the United States, which has no formal diplomatic relations with North Korea.

Chancellor Park said roughly 60 U.S. citizens come to PUST each semester, but now "there's less than that".

North Korea's official news agency KCNA said Kim was detained for "hostile acts", without elaborating. Tony Kim, another professor who worked at PUST, was arrested two weeks earlier for a similar reason.

On Thursday, KCNA said it was its sovereign right to "ruthlessly punish" U.S. citizens it has detained for crimes against the state, saying U.S. media's description of such arrests as a bargaining ploy was "pure ignorance".

"Recent Americans detained are being interrogated by relevant legal authorities for criminal acts against the Republic," KCNA said, without mentioning the two men by name.

"It is an exercise of the legitimate right of a sovereign state to deal with the criminals according to its law ... The DPRK will detect and frustrate every anti-DPRK plot of the dishonest hostile elements and ruthlessly punish the criminals and thus reliably defend its state and social system."

DPRK is short for the North's formal name, the Democratic People's Republic of Korea.

A spokesman for the university which opened in 2010, said the arrests of the two faculty members were "not connected in any way with the work of PUST".

The detentions came amid tensions on the Korean peninsula over North Korea's pursuit of nuclear weapons in response to what it says is a threat of a U.S.-instigated war.

The White House said on Monday the latest reported detentions were "concerning" and the State Department was working with the Swedish embassy in Pyongyang to seek their release. (For a graphic on Americans detained by North Korea, click http://tmsnrt.rs/2r5xYpB)

SETTING CHRISTIAN EXAMPLES

Two years before he was detained, Kim Hak Song raised money for his trip to North Korea from members of the Korean-language Sao Paulo Oriental Mission church in Brazil, according to his post on the church's website.

"I've committed to devoting my last drop of blood to this work," he wrote.

Kim, a Chinese-Korean and naturalized U.S. citizen, had been doing missionary work in China before joining PUST, according to Korean-language church websites.

Kim's detention makes him the fourth American in North Korean custody. (See Graphic: http://tmsnrt.rs/2pmE3ks)

In March last year, North Korea sentenced U.S. college student Otto Warmbier to 15 years of hard labor for the alleged theft of a propaganda poster. South Korean-born Kim Dong Chul, a naturalized U.S. citizen, was convicted a month later and sentenced to 10 years hard labor – shortly after Washington levied more sanctions against Pyongyang in response to a missile test in February of that year.

Founded by Korean-American evangelical Christian James Kim, PUST spends roughly $2 million annually on operating expenses, the school said in a statement. Much of it comes from the Korean diaspora in the United States, along with churches in South Korea and private foundations and philanthropists.

PUST has 500 undergraduate students and 60 graduate students in mostly three departments - electronic and computer engineering, international finance and management and agriculture and life sciences.

The school recruits many of its teachers from Korean churches and Christian colleges in the United States. Faculty receive no income or stipends from the university, but do get housing and cafeteria meals.

PUST has a sister institution across the border in northern China called the Yanbian University of Science and Technology (YUST).

Tony Kim, the first PUST faculty member to be detained, was listed as a professor of accounting at YUST on its website. He moved to PUST in 2006, four years before the university opened, to "take care of financial matters," according to the newsletter seen by Reuters, which was recruiting teachers for the school.

PUST also recruits teachers via social media and at overseas universities and churches, via the YUST PUST Foundation, its U.S.-based charity arm according to the foundation's website, and former teachers.

The foundation raised just over $1.1 million in 2015 and has brought in $4.5 million since 2011, according to tax filings.

OPENING MINDS

The recent detentions are not the first time teachers from the school have attracted unwanted attention.

A 2014 memoir by Korean-American Suki Kim, compiled while she was an English professor at PUST, said the faculty was constantly monitored.

Former PUST teachers said her account was exaggerated. "The reality is much softer and friendlier," one said.

Not all PUST teachers are religious.

Will Scott, a research fellow at the University of Michigan who taught at PUST in 2013 and 2015, told Reuters he felt welcome in the community despite being an atheist.

"The foreigners would have a service on Sundays for themselves, but wouldn't talk about it around the students or in class," said Scott, who taught a software engineering class to PUST students.

Most of the students have little access to outside information in the very closed society they live in.

"You see students minds opening, world view expanding, curiosity rising and ethical aspects of life coming into the picture," said a former PUST teacher, who declined to be named.

Abraham Kim is the executive director of the Chicago University Bible Fellowship, which donated $30,000 in 2013 to PUST's campaign to build a new medical school.

He said that while the volunteers "can't directly preach the word of God, we can indirectly influence the people there by being good Christians".

(Additional reporting by Heekyong Yang and Jack Kim in SEOUL, Joseph Ax and Angela Moon in NEW YORK, Alex Dobuzinskis in LOS ANGELES, Editing by Bill Tarrant, Soyoung Kim and Alison Williams)