Symantec is warning of a sharp jump in online attacks that appear to be targeting a recently patched bug in Microsoft's Windows operating system, an analysis that some other security companies disputed Friday.

Symantec raised its Threat Con security alert level from one to two because of the attacks, with two denoting "increased alertness." But other vendors, including Arbor Networks and McAfee, said they were seeing no such activity.

The attacks spotted by Symantec target a flaw in the Windows Server Service that Microsoft says could be exploited to create a self-copying worm attack. Late last month, Microsoft took the unusual step of rushing out an emergency patch for the bug after it saw a small number of online attacks that took advantage of it.

Since then, security experts and Microsoft have said that the attacks have not been widespread, but that may now be changing, according to Symantec.

The security vendor said it had seen a "dramatic rise" in attacks targeting TCP (Transmission Control Protocol) port 445. A TCP port is a number assigned to packets of data sent over the Internet to help computers know what program should be processing the information. Web browsers, for example, typically use port 80. Port 445 is one of two ports used to connect with the Windows Server Service.

This activity "appears to be related to the exploitation" of the Windows Server bug, Symantec said in a note on its Web site.

Most firewalls block port 445, as well as the other port used by Server Service, port 139, but Symantec said Windows users should still make sure they've applied the MS08-067 patch for the bug.

Attacks on the bug had focused previously on Chinese versions of Windows, but the latest attacks target English versions, Symantec said.

Arbor Networks disputed Symantec's interpretation, saying, "we’re not seeing this rise, not on TCP port 445 and not on TCP port 139. Looking over the last month we don’t see this rise in MS08-067 attacks that would raise any alarms for us," in a Friday blog posting.

Both McAfee and Microsoft echoed those sentiments.

"Microsoft continues to see limited, targeted attacks that attempt to exploit this vulnerability," Microsoft said in a statement.

On the other hand, the Research and Education Networking Information Sharing and Analysis Center, which monitors research and university networks, reported that it had seen a bump in port 445 activity.

Security experts say that if criminals are targeting specific networks with their attacks, it could account for the discrepancy.