Norway's security service on Tuesday said it arrested a Norwegian man suspected of trying to spy on behalf of China.
The suspect was detained Monday at Oslo's international airport as he returned from China, Thomas Blom, a spokesman for the service's counter-espionage unit, told reporters.
An Oslo court placed him in four-week pre-trial detention with a ban on communicating with the outside, and with the first two weeks to be spent in isolation.
According to the Norwegian media, the suspect is a man in his thirties, living in Oslo and politically active in a local branch of Norway's incumbent Labour Party.
A lawyer for the man, whose identity has not been revealed, said he denied the charges against him.
"He says he is innocent and that he is not an agent for China," the lawyer, Marius Dietrichson, told AFP.
He said he could not provide further details because of the sensitive nature of the accusations.
The security services have not provided details on what information the suspect might have provided to China.
"We are in a preliminary and extremely sensitive phase," Blom told reporters as he left the Oslo court.
"We are at the very preliminary stage where we are gathering the evidence."
In an email to AFP, the Chinese embassy in Oslo said some Norwegian agencies had made "irresponsible accusations" and "propagated the 'China threat'".
"We believe that the Norwegian side will base its stance on facts, remain vigilant against attempts to damage bilateral relations, and take concrete actions to maintain the healthy and stable development of relations between our two countries."
In their annual evaluation of the risks facing Norway, the country's security services said China and Russia were the two major threats in terms of espionage.
The growing threat from China is due to "the deteriorating relations between China and the West, China's desire for greater control over supply chains and its interest in the Arctic," the report said.
Norwegian authorities are currently seeking to prevent the sale of the last piece of private property in the Arctic archipelago of Svalbard.
According to a lawyer representing the owners, a Chinese group had expressed interest in the 60 square-kilometre (23 square-mile) domain.