Four employees of global mining giant Rio Tinto in Shanghai have been detained, apparently on suspicion of espionage and stealing state secrets.

Shanghai's State Security Bureau yesterday confirmed to China Daily that the four were being investigated by the bureau. The bureau would not give a reason why they had been detained.

Three of the four being detained since July 5 are Chinese citizens. The fourth is an Australian named Stern Hu, general manager for China operations at Rio Tinto's iron ore division.