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Three Chinese citizens have been charged by a US court with hacking attacks on international corporations.

The three, said to run a cyber security firm in China, targeted Moody's Analytics, Siemens and GPS maker Trimble, the indictment says.

They are alleged to have used email phishing scams and malware to try to steal business secrets.

The three are not in custody and it is not clear if they are still in China.

The federal indictment, unsealed in Pittsburgh, Pennsylvania, names them as Wu Yingzhuo, Dong Hao and Xia Lei and says they run a cyber security services firm in the southern Chinese city of Guangzhou.

It alleges that between 2011 and this year they launched "co-ordinated and unauthorised" cyber attacks, stealing emails, business and commercial information and sensitive employee information.

Prosecutor Soo C Song said arrest warrants had been issued for the three. She said there was no suggestion that the hacking was state-sponsored.

The Chinese company has not commented on the charges.

Prosecutors say the indictment was filed in September and that the Chinese government is aware.

Cyber attacks have been a bone of contention between the US and China for some years.

In August a Chinese man was arrested in Los Angeles accused of selling malware that was used to hack into US companies.

That malware has been implicated in the 2014 hack attack on the US federal Office of Personnel Management (OPM).

Information on a huge number of US workers employed in sensitive or secret work was stolen in that data breach.

In 2016, a Chinese businessman was sentenced to nearly four years in jail in the US after admitting hacking sensitive information from US defence companies.

The Chinese government has repeatedly denied any involvement in hacking foreign companies or governments.