Dadong Border China-North Korea
North Korean soldiers stand guard behind the national flags of China, front, and North Korea on a boat anchored along the banks of the Yalu River, near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, on June 10, 2013. Reuters/Jacky Chen

(Reuters) - A Korean-American who runs a Christian NGO in a Chinese city on the border with North Korea is being investigated by Chinese authorities and has had his bank accounts frozen, a source with direct knowledge of the case told Reuters on Thursday.

Peter Hahn, a naturalized U.S. citizen, has been under interrogation by Chinese authorities for the last three weeks and is not permitted to leave the country, said the source, who requested anonymity, citing the sensitive nature of the case.

The source did not know what prompted the probe, which coincides with an investigation of a Canadian Christian couple who run a coffee shop in the nearby city of Dandong on suspicion they stole military secrets.

Several people working in the region, or who are in contact with those that do, said Hahn's case appeared to be part of a wider sweep of Christian-run NGOs and businesses along the Chinese side of the border with North Korea.

While China can be suspicious of Christian groups and President Xi Jinping has launched a wide crackdown on underground churches, foreign missionaries usually operate without too much harassment.

Hahn runs a school for ethnic Korean children in the Chinese city of Tumen. Through his Tumen River Area Development Initiative (TRADI) NGO, he also operates several humanitarian projects and joint venture companies inside North Korea, including a local bus service in the Rajin-Songbon (Rason) Special Economic Zone.

The school declined to comment when asked about the case, and Tumen police could not be reached for comment.

A spokesman at the U.S. embassy in Beijing said he could not provide any information on the matter. Marie Harf, a spokeswoman for the U.S. State Department in Washington, said she was aware of the reports about Hahn's detention but could not comment further for reasons of privacy.

Hahn's company cars had been confiscated and his bank accounts frozen, the source said, adding that his NGO's humanitarian food shipments to North Korea had been suspended following the freeze.

"OPENLY CHRISTIAN"

The source said that Hahn was a Christian and was open about his faith. A description on its website said Hahn founded the NGO in 1997 to help North Korean refugees in Yanji, the capital of China's autonomous ethnic Korean region, a short drive from Tumen, where Hahn is based.

"TRADI's main goal is to sacrifice themselves for the ministry of God," a description of the NGO's vision on its website says. "Through the bridge that TRADI built around the community of Tumen and North Korea, we hope to break down the wall of oppression, imprisonment, and injustice."

North Korea espouses freedom of religion, but it is ranked as one of the world's most oppressive regimes in terms of such freedom, and severely punishes citizens who veer from a state-sponsored ideology that venerates its leaders.

It was not immediately clear why China, North Korea's main ally and economic benefactor, was cracking down on missionaries in the region, but experts said it had cooperated with North Korea in the past along the border.

"North Korean security forces and diplomatic officials are working particularly hard this calendar year to combat growth in religious activities along the border," said Christopher Green of the Seoul-based Daily NK website, quoting repeated reports from sources both inside North Korea and in cities on the Chinese side of the border.

Reuters was unable to independently verify the reports from inside isolated North Korea.

Christian activists focused on China said the two countries had been collaborating closely, despite an apparent dip in relations between Beijing and Pyongyang.

"In recent years they have not always seen eye to eye, but it's a common interest for both sides to crack down on the border, particularly refugee issues," said Bob Fu of China Aid, a U.S.-based Christian advocacy group.

China's porous border with North Korea, which snakes through mountains, is a thriving hub of cross-border black market trade as well as a route taken by North Korean refugees fleeing economic hardship and religious and political persecution.

"JERUSALEM OF THE EAST"

"Pyongyang was once known as the Jerusalem of the East," said David Alton, chairman of the UK parliament's All-Party Parliamentary Group on North Korea, referring to how the city was a thriving center for missionaries long before Korea was divided.

"North Korea's leadership is well aware that below the surface, secret religious faith remains deep in the hearts of many North Koreans," he told Reuters.

"Their fear of Christianity has led to suppression, persecution (and) cruel incarceration."

Many missionaries are drawn to the Rason SEZ, where foreigners, including U.S. citizens, can gain a residence permit and set up joint venture companies with the local government.

While some foreign Christians in sensitive border areas are relatively open about their faith, others are wary of being monitored by North Korea and China and use phrases like "meeting place" to describe underground or house churches.

Buses operated by Hahn's NGO with a North Korean partner in Rason bear a logo resembling a fish, a secret symbol used by early Christians persecution in the Roman Empire.

U.S. missionary Kenneth Bae, sentenced by Pyongyang last year to 15 years hard labor for attempting to overthrow the state, operated businesses in Dandong and used his tour company, Nation Tours, to take foreign missionaries across the border into North Korea.

SOUTH KOREAN FUNDS

Beyond tours, bakeries that produce small, nutritious buns that are distributed to school children and orphans in North Korean border cities are often set up by missionary groups in China, or in North Korea in cooperation with the North Korean authorities.

The bakeries distribute much-needed food to impoverished North Korea, but critics argue such humanitarian aid is often deeply intertwined with religious conversation and salvation.

Hahn, who is based in California but has a residence permit for Rason, also runs a bakery, attached to the school, which has produced bread for North Korea since 2003.

Deliveries of bread from the bakery to North Korean children had been delayed by the freeze of Hahn's funds, the source said.

A proportion of Hahn's funding came from churches in South Korean. His school received money from Pohang Baptist Church in South Korea, according to a letter from the school's vice principal on the church's website.

The investigation into Canadians Kevin Garratt and his wife Julia Dawn Garratt came a week after Canada took the unusual step of singling out Chinese hackers for attacking a key computer network and lodged a protest with Beijing.

In response, China accused Canada of making irresponsible accusations that lacked credible evidence.

"With the Garratts, that was tit-for-tat with what happened in Canada. Peter Hahn is a different issue, I think it's more related to his faith and the work he was doing," said David Etter, who was recently forced to close his Christian-run Western restaurant in Yanji, citing a lack of customers.

"He was very open about his faith and why he was doing what he was doing," he said.