Windows Mobile smartphones have been targeted by hackers who are distributing gaming apps with hidden malware, security firm Lookout warned.

Hackers are distributing "3D Anti-Terrorist” and "PDA Poker Art" games, which are pre-packed with viruses. Once the app is installed, the virus becomes active and starts dialing up premium-rate numbers across the globe.

Viruses dial up premium numbers like in Somalia and the South Pole, but Windows Mobile phone owners may not detect the problem until they get phone bills higher than usual.

Auto-dialer scams are becoming more and more common these days. According to Lookout, the number of phones afflicted with viruses in the US jumped from four per hundred to nine per hundred in the last six months.

Microsoft said it was aware of the problem and started investigating it. It advised Windows Mobile phone users to visit http://www.microsoft.com/protect/ to get advice on how to protect their device.