China has said it had evidence proving detained Rio Tinto staff stole state secrets, as the affair threatened to boil over into a diplomatic row with Australia summoning the Chinese ambassador.

Stern Hu, the Australian national in charge of the Anglo-Australian mining giant’s Shanghai office, faces criminal charges for stealing state secrets for foreign countries, Chinese foreign ministry spokesman Qin Gang said.

“Competent authorities have sufficient evidence to prove that they have stolen state secrets and have caused huge loss to China’s economic interest and security,” Qin told reporters.

Chinese authorities collected evidence against Hu before arresting him and other Rio Tinto staff on Sunday, Qin said. Shanghai authorities earlier confirmed the others were three of Hu’s Chinese colleagues.

Qin said Hu’s issue should be treated as an isolated incident and that China did not want it to affect trade relations between China and Australia.

“It’s improper to exaggerate this individual case or even politicise it, which will be no good to Australia,” Qin said.

Australia’s Mandarin-speaking Prime Minister Kevin Rudd rejected opposition calls on Thursday for him to discuss the case with Chinese President Hu Jintao.

“The key thing is not for politicians … to begin trying to politicise issues like this but rather let’s get on with the practical business of working with the very difficult case on the ground,” Rudd told reporters in Rome.

However the Australian foreign ministry announced that the acting Chinese ambassador had been called in to discuss the issue, with a lack of consular access to Hu one of the issues raised.

Following the meeting, Chinese authorities said Australian consular officials would be able to meet Hu on Friday, the foreign ministry said in a statement.

Foreign Minister Stephen Smith earlier said there was no suggestion that Australian passport-holder Hu’s detention was linked to Rio’s fraught iron ore talks with Beijing or its recent decision to snub a massive Chinese cash injection.

“I’ve seen nothing which would cause me to in any way believe or form the conclusion… that the events associated with Mr Hu are in any way related to Rio Tinto’s commercial activities,” he said.

Smith said the spying claims came as a “surprise.”

The four Rio Tinto executives are being held by China’s secretive state security ministry, which handles counter-espionage operations.

A Rio spokesman told AFP it was not giving out any more information on the case due to its “sensitive” nature, but the company also indicated it had been taken by surprise.

“We are not aware of any evidence that would support such an investigation,” the company said in a statement.

The incident has cast a shadow over Australia’s relations with one of its largest trading partners, in which Rio Tinto plays a key role as a major supplier of iron ore and other raw materials to China’s growing economy.

Last month, the debt-laden miner rejected a 19.5-billion-dollar cash offer from China’s Chinalco after deciding that rising commodity prices made a rights issue and joint venture with BHP Billiton more attractive.

The miner has also spearheaded difficult talks with China over new iron ore contracts, which missed a key deadline at the end of June.

Australian media have reported speculation that the detentions were linked to alleged manipulation of the iron ore market, while the Chinese press has accused Rio of withholding products to drive up prices.

China’s state-controlled Securities Times also quoted unnamed “industry insiders” as speculating the detentions may have been over suspected involvement in bribery.

“I will be fascinated to hear what evidence they have that Mr. Hu is a spy,” opposition senator Barnaby Joyce told Sky News.

“It sounds awfully like a trumped-up charge to me.”