The games — 3D Anti-Terrorist and PDA Poker Art — are available on sites that provide legitimate software for mobile devices, according to John Hering, CEO of San Francisco-based security firm Lookout. The games are bundled with malicious software that automatically dials premium-rate telephone services in Somalia, Italy and other countries, sometimes ringing up hundreds of dollars in charges in a single month.

The services are run by the programmers who built the tainted software, Hering said on Friday. Victims generally do not realise they have been infected until they get their phone bills and see hundreds of dollars of unexpected charges for those premium-rate services, he said.

Hackers are increasingly targeting smartphone users as sales of the sophisticated mobile devices have soared with the success of Apple Inc’s iPhone and Google Inc’s Android operating system.

Microsoft said it was aware of the problem and investigating it.

“As always, Microsoft continues to encourage customers to follow all of the steps of the ‘Protect Your Computer’ guidance of enabling a firewall, applying all software updates and installing anti-virus and anti-spyware software,” said a spokesman.

Microsoft said customers should visit www.microsoft.com/protect/ for guidance on protection from viruses.