An upsurge of hacker activity on the internet is causing alarm among computer experts.
Microsoft released a fix for the problem two weeks ago
US Government experts say they have received reports that hackers are looking for ways to take advantage of a critical flaw in Microsoft's Windows.
"An internet-wide increase in scanning for vulnerable computers over the past several days reinforces the urgency for updating affected systems," said the US Department of Homeland Security.
It warned that the flaw in Windows could soon be used by a computer virus to disrupt traffic on the net and potentially affect millions of machines worldwide.
The vulnerability affects almost all versions of Microsoft's Windows operating system. It affects the technology used to share data files across computer networks.
It involves a category of vulnerabilities known as buffer overflows, which let intruders run malicious code on affected machines, giving them total control over the systems.
Microsoft warned of the flaw two weeks ago and released a fix for the problem that can be downloaded for free from its site.
Microsoft Windows NT 4.0
Microsoft Windows NT 4.0 Terminal Services Edition
Microsoft Windows 2000
Microsoft Windows XP
Microsoft Windows Server 2003
Concern about the threat to computers worldwide has been growing.
Instructions about how to exploit the flaw in Windows were released on the internet several days ago.
Since then, experts have noticed a rise in the hacker activity, with intruders looking for computers which were open to attack.
"Because of the significant percentage of internet-connected computers running Windows operating systems and using high-speed connections, the potential exists for a worm or virus to propagate rapidly across the internet," warned the Department of Homeland Security.
It has urged computer users to download a free patch that has been available on Microsoft's site since 16 July.
FBI spokesman Bill Murray said the bureau was studying several hacker tools designed so far and was
highly concerned about a wide-scale internet attack.
"We implore the private sector, both business and home users, to visit the Microsoft website and install the patches and
mitigations necessary to prevent this from creating a negative effect on the internet as a whole," he said.
Some experts have been more cautious about the immediate threat to the net.
Oliver Friedrichs of anti-virus firm Symantec said the widespread attacks might not occur soon because hackers still need to resolve important glitches in their own attack tools.
"The exploit needs to be perfected," he said. "The effort applied to the exploit is certainly increased, but we're not sure if that's indicative of when we might see a widespread threat. People certainly need to be aware of this."