Computer viruses aimed at Google's Android smartphone operating system have surged, AhnLab said on Wednesday. Android is used by most Korean smartphone makers such as Samsung Electronics, LG Electronics, and Pantech.

The computer security firm has detected a total of 2,251 malicious codes lurking in the Android operating system since July, up more than 17 times the 128 in the first half of this year. The biggest lot was 1,637 Trojan horse viruses, which inflict damage on users by connecting with paid services without their knowledge, followed by 743 spyware codes, which steal users' personal information.

AhnLab attributed the spread of the viruses to the proliferation of tools that make them easily available on the Internet. Many hackers now can use malicious codes made by other hackers without taking the trouble to develop their own.

Another reason is that more Android smartphones are sold, making them attractive targets for hackers. Android smartphones now account for about 70 percent of the Korean market.

Most of the malicious virus codes were found in unofficial apps marketplaces. Hwang Mi-kyung of AhnLab said no virus has been found in Google's official apps marketplace since July.

Android phone users can download apps from unofficial private marketplaces, in addition to official markets operated by Google or domestic mobile service providers.