The son of a Canadian couple under investigation for alleged spying in China called Wednesday for Ottawa to help prevent charges being laid against his parents.

Kevin Garratt and Julia Dawn Garratt "are suspected of gathering and stealing intelligence materials about, among other things, China's military objectives and important national defence research projects", China's foreign ministry said in a faxed statement Wednesday.

Such activities "harm China's security", it added.

The Christian couple run a coffee shop in the northeastern Chinese city of Dandong, on the border with North Korea.

"I really hope the Canadian government puts some resources into it and really tries to resolve it quickly," their son Simeon Garratt told AFP by phone from Vancouver, where he lives.

"Because I think the longer it goes on it's exponentially worse for my parents."

The cafe, Peter's Coffee House, overlooks the Yalu river and the Friendship Bridge linking China and North Korea, and the "T" in its name is in the shape of a crucifix, with a backdrop resembling a stained-glass window.

The couple, who have been out of contact with family since late Monday, have been active in helping send humanitarian aid to impoverished North Korea, their son said, describing them as "openly Christian".

In an audio file posted last year on the website of the Terra Nova church in Surrey, British Columbia, Kevin Garratt told the congregation that they have supported North Korean Christians who return to their country "to preach the gospel".

The file was no longer available Wednesday.

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.

China's definition of state secrets can be very broad while North Korea is deeply suspicious of Christian proselytising activities, punishing them harshly.

Garratt said that his brother Peter, who studies in China, was called in by authorities in Dandong and told his parents were being held at an undisclosed location.

The Canadian Embassy in Beijing said Wednesday that "consular officials are providing assistance" to the pair and reiterated an earlier statement that it is "monitoring developments closely".

- Student 'sold intelligence' -

In a separate espionage accusation, state media reported Wednesday that a student has been detained for selling "intelligence" to foreigners for more than $32,000.

The student, surnamed Chang, is an aerospace graduate student at Harbin University in northeast China's Heilongjiang province, according to the official Xinhua news agency.

Police on Tuesday authorised the arrest of Chang, who was found to have "gathered intelligence" for "foreign personnel" more than 50 times over nearly two years, Xinhua said.

The Garratts' case was front-page news Wednesday in the Chinese-language edition of the Global Times tabloid, which has close ties to the ruling Communist Party.

"Canadian 'husband-wife spies' cause speculation," read the headline, with a sub-heading reading that they had "frequent contact with Western reporters".

The paper's English-language edition said in an editorial that the probe may counter "an impression that China is a country engaging in rampant espionage activities compared to other countries".

"China has sufficient evidence to prove that foreign intelligence agencies engage in a large amount of espionage activities in China," it said.

The investigation against the Garratts came a week after Canada accused Beijing of hacking into the computers of its research and development arm, a claim China called "groundless".