SAN FRANCISCO, US - COMPUTER-SECURITY top guns worldwide watched warily as the dreaded Conficker worm squirmed deeper into infected machines with the arrival of an April 1 trigger date.

Nothing dramatic happened - which is precisely why people should stay worried.

"Planes are not going to fall out of the sky and the Internet is not going to melt down," said threat analyst Paul Ferguson of Trend Micro computer-security firm in northern California.

"The big mystery is what those behind Conficker are going to do. When they have this many machines under their control, it is kind of scary. With a click of a mouse they could get thousands of machines to do whatever they want."

A task force assembled byMicrosoft has been working to stamp out the worm, referred to as Conficker or DownAdUP, and the US software colossus has placed a bounty of US$250,000 (S$380,000) on the heads of those responsible.

The worm was programmed to modify itself yesterday to become harder to stop. Conficker task-force members tracking Internet traffic in Asia and Europe after clocks struck April 1 said there was no sign that the worm was doing anything other than modifying itself to be harder to exterminate.

Conficker had been programmed to reach out to 250 websites daily to download commands from its masters, they said, but yesterday it began generating daily lists of 50,000 websites and reaching out randomly to 500 of those.

The hackers behind the worm have yet to give specific orders to the estimated one to two million infected computers worldwide. Said Mr Ferguson: "There is this loaded pistol looming large out there even if no one has pulled the trigger."

The worm takes advantage of networks or computers that have not kept up to date with security patches for Windows RPC Server Service.

Malware could be triggered to steal data or turn control of infected computers over to hackers amassing "zombie" machines for botnet armies. -AGENCE FRANCE-PRESSE