Governments across the globe have been urged to prepare for a second denial of service (DoS) attack after an earlier incident brought down several official web sites.

Government sites in the US and South Korea were subjected to DoS attacks earlier this week, apparently originating from North Korea. Experts believe that the attacks are working to a schedule, and have warned that further incidents could occur later today.

Rik Ferguson, senior security advisor at Trend Micro, said that the attacks are being launched from an estimated 50,000 infected machines, most of which will not have been purged of the offending malware.

There is also the possibility that those responsible for the attacks used only a part of the available botnet, or that the targets have shifted.

"There are a large number of affected machines, and they will have a list of targets. They started attacking the US and then moved to South Korea. They could still shift their targets," said Ferguson.

Security vendor Symantec, meanwhile, is already monitoring an attack on the US and South Korea, and suggested that its scale and scope may widen to affect millions of computer users.

"Initially, it was reported that the attack leveraged more than 50,000 computers. The size of the botnet used for this distributed DoS attack is only a fraction of the one that is still being created by Downadup/Conficker, which was estimated at a few million machines at its peak," said the firm in a security note.

"If a system is infected, the user may not experience any performance slowdown. However, users trying to visit the impacted sites may experience significant slowdown and inability to access the sites."

All computer users are advised to shore up their own defences in the light of such attacks.