Nearly six weeks after being detained by the Shanghai State Security Bureau for suspected espionage activities, Shanghai procuratorate authorities have now formally arrested four Rio Tinto employees.

The four, including Australian executive Stern Hu, have been charged with infringing business secrets and bribery, the Xinhua News Agency reported yesterday.

The arrest came shortly after Hu received his second consular visit from Australian officials.

Chinese legal experts said the case was "complicated" but insisted Hu had been treated in line with normal legal procedures.

Wang Minyuan, an expert on legal procedure at the Chinese Academy of Social Sciences, told China Daily that police and prosecutors' dealings in the Rio Tinto case had been legal.

"China's criminal law of procedure stipulates that the police normally must arrest or release detainees within one month from the start of their detention. However, detention may be prolonged if prosecutors need to conduct additional investigation," he said.

The Criminal Procedural Law stipulates that prosecutors may apply for additional time for investigations up to twice, which would allow police to detain suspects for one additional month each time.

The same law stipulates that legal representation must be given to suspects if the case is connected to State secrets.

Rio Tinto, meanwhile, has moved employees out of its Shanghai office following the detention and arrest of Hu and the three others, said Sam Walsh, head of London-based Rio's iron ore unit, according to Bloomberg yesterday.

Walsh said some non-Chinese employees in Shanghai had moved to Singapore.

China is Australia's second-largest trading partner.