Bonn, Germany  - A computer virus which has spread worldwide began mutating overnight, German government computer experts warned Thursday.

The Conficker worm uses the internet to install new functions on millions of infected computers, the government's BSI information- technology agency said in Bonn. The creators of the virus intend to create a botnet or network of zombie computers to do their bidding.

Criminals have set up botnets in the past to distribute spam email. BSI said it was still analysing the new Conficker code, which spreads thanks to a security gap in the Windows operating system.

Microsoft patched this last October but millions of computer owners have not followed recommendations to update their software against the loophole.

BSI advised computer users to update their Windows, web browsers, Acrobat Reader and Flash software and use firewalls and anti-virus software to defeat the virus.