China is investigating a Canadian Christian couple for alleged espionage, state media and their son said Tuesday, a week after Ottawa accused Beijing of cyber-spying.

Kevin Garratt and Julia Dawn Garratt "are under investigation for suspected theft of state secrets about China's military and national defence research," the state-run Xinhua news agency reported.

The couple first came to China decades ago and now run Peter's Coffee House in Dandong, on the border with North Korea, their son Simeon told AFP.

The cafe overlooks the Yalu river and the Friendship Bridge linking the two countries, and the "T" in its name is in the shape of a crucifix, with a backdrop resembling a stained-glass window.

China's definition of state secrets can be very broad, and the Xinhua report did not specify what was allegedly stolen, adding only that the probe was being conducted by the State Security Bureau in Dandong.

The Dandong region is a sensitive military area for China, and the border crossing is a key trade lifeline for nuclear-armed, diplomatically isolated North Korea.

Beijing sent troops to fight for the North during the 1950-53 Korean War and has long been Pyongyang's key ally.

The Dandong area is also a focus for foreign Christian groups, including from South Korea, with some working to assist North Koreans who secretly cross the border to escape from hardship and repression in their homeland.

Garratt described his parents as "openly Christian" and said they had visited the impoverished North several times.

They were involved in sending humanitarian supplies to "help basically what they feel is a group of people that have been severely neglected".

His parents had been unreachable since Monday evening, added Garrett, 27, who runs a software company in Vancouver.

"I just think it's crazy," he said of the investigation.

"It sounds like something somebody made up," he added. "I really don't know why. It's just so absurd."

On its website the coffee shop describes itself as "only metres from the border of North Korea", and "the perfect stop off while en route to or returning from the Hermit Kingdom".

"Sorry, today we are closed," read a sign in the shop window Tuesday. "See you soon!" it added, followed by a smiley face.

- 'God said go to Dandong' -

In an audio file posted on the website of the Terra Nova church in Surrey, British Columbia, Kevin Garratt tells the congregation: "We're China based, we're North Korea focused, but we're Jesus centred."

"God said, in a prayer meeting, go to Dandong and I'll meet you there, and he said start a coffee house," he said in a guest sermon dated November last year.

"We serve the best coffee on the border... and we do some other things too.

"We're trying to reach North Korea with God, with Jesus, and practical assistance."

North Koreans regularly stay at a "training house" outside Dandong, he said, and "99 percent of the people we meet go back to North Korea, because they have to preach the gospel in North Korea, because God has compelled them to go".

North Korea is deeply suspicious of Christian proselytising and metes out harsh punishment. It announced in May that a South Korean missionary was sentenced to hard labour for life after he was accused of espionage and setting up an underground church.

The Canadian Embassy in Beijing said it was aware of reports Canadians were being investigated.

"We are gathering information and monitoring developments closely," it said in an email, but declined to release further information, citing privacy issues.

News of the investigation came just a week after Canada accused China of hacking into the computers of its research and development arm.

"Recently, the government of Canada, through the work of the Communications Security Establishment, detected and confirmed a cyber intrusion on the IT infrastructure of the National Research Council of Canada by a highly sophisticated Chinese state-sponsored actor," a government statement said last Tuesday.

Canada did not provide details on the nature of China's alleged cyber attack.

Beijing dismissed the claim as "groundless speculations and accusations" in a statement issued by its embassy in Ottawa.

Simeon Garratt told AFP that he, his brother and two sisters were "confused" and "trying to get a handle on things".

His parents came to China in 1984, have lived in Dandong for about seven years and are fluent Chinese speakers, he said, adding his mother is British-born.

Over the years they have taught English, run a consulting business, started a kindergarten and helped Chinese orphans find homes in the West.

"Obviously, they've always had a huge passion for China and just helping the Chinese people in general and that's really been their goal everywhere in China they've been," he said.