China has released on bail a Canadian woman under investigation for undermining state security while her husband has been held on spying charges, Beijing said Thursday, six months after they were detained.

Kevin Garratt and Julia Dawn Garratt, a Christian couple who ran a coffee shop in the Chinese city of Dandong, on the border with North Korea, were seized last August, prompting an international diplomatic row.

Chinese foreign ministry spokesman Hong Lei said Julia Garratt was "granted bail pending trial" on Tuesday by national security authorities in Dandong.

Kevin Garratt was placed in criminal detention the same day "on the charge of spying and stealing state secrets", Hong said, adding that both Garratts are suspected of "committing activities undermining China's state security".

"The relevant case is under investigation. The Chinese government will protect their lawful rights and interests in accordance with law," Hong added.

A spokesperson for the Dandong public security bureau said he had no information on the case.

The Garratts first moved to China in the 1980s and have been active in helping send humanitarian aid to impoverished North Korea, where authorities are deeply suspicious of Christian proselytising and punish it harshly.

Kevin Garratt has told congregations in Canada: "God said, in a prayer meeting, go to Dandong and I'll meet you there, and he said start a coffee house.

"We're China-based, we're North Korea-focused, but we're Jesus-centred."

- 'Mission Impossible' -

The probe into the couple was announced one week after Canada accused China of hacking into its research and development arm, prompting accusations that Beijing was investigating them as retaliation against Ottawa.

It has strained ties with Canada, which threatened to cancel a visit to Beijing by Prime Minister Stephen Harper in November over the issue, although he ultimately relented.

Beijing has rejected Ottawa's cyber-espionage claim as "groundless speculations and accusations" and denies that the Garratts were detained as punishment for the allegation.

Yet the ruling Communist Party has previously taken action against foreign nationals during heated international spats, spurring criticism that it has sought to use such individuals as leverage in larger political battles.

In 2010, with Beijing infuriated by its longtime rival Tokyo's detention of a Chinese fishing captain in a disputed area of the East China Sea, Chinese authorities detained four Japanese construction workers in the northern province of Hebei.

The men -- the last of whom was released 20 days later -- were accused of violating Chinese law by filming a military site in a prohibited area.

Charles Burton, an expert on Sino-Canadian relations at Brock University in Ontario, said attempted retaliation against the cyber-spying charges was "the only explanation that makes sense to us" for the Garratts' detention.

Under President Xi Jinping China has mounted a crackdown on Christian missionaries, but Burton said: "The idea that they are involved in such a serious crime seems incredible to us.

"It sounds like something out of Mission Impossible. Nobody believes that there's any possibility."

"Canadians in general believe that Canada should stand up to China, and in particular for the rights of Canadians caught up in geopolitical games not of their own making," he added.

- 'Sense of urgency' -

Authorities have ordered Julia Garratt to remain in mainland China for one year, barring her from leaving the country, according to a statement released by the family through their Beijing-based lawyer, James Zimmerman.

Kevin Garratt has been moved to a detention centre "at an unknown location", the statement said, adding that the couple had not been formally arrested, no charges filed, and "no evidence of any crime" provided.

Simeon Garratt, the couple's son, previously said that Chinese authorities had allowed Canadian consular officials to visit his parents every two weeks and that both Garratts were subject to frequent interrogations.

Family members of the couple welcomed the release but said they remained "deeply concerned" that Kevin Garratt was still being held.

"We call upon the Chinese government to ensure that this matter is handled with transparency and due process as required by Chinese law and fundamental international standards," Zimmerman said in a statement, calling for Beijing and Ottawa "to resolve this matter involving diplomatic means with a sense of urgency".

Canadian foreign affairs spokesman John Babcock hailed Julia Garratt's release but said Ottawa "remains very concerned with the detention of Mr Garratt".