Experts are advising computer users with renewed urgency to apply a free repairing patch that Microsoft has offered on its Web site since July 16, when it acknowledged that the flaw affected nearly all versions of its flagship Windows operating system software.

The Homeland Security Department cautioned Wednesday that hackers in recent days have successfully tested new tools to seize control of such vulnerable computers over the Internet, stealing data, deleting files or eavesdropping on e-mails. The government also said it had detected an "Internet-wide increase in scanning" for victim computers.

Security companies guarding government and corporate networks have identified sporadic break-in attempts worldwide using such tools and monitored hackers in discussion groups and chat rooms exchanging tips about how to improve the effectiveness of their programs.

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