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Skype Ltd. early Thursday blamed an unspecified "software problem" for an outage that might make the voice-over-IP service unavailable for as long as 24 hours.

"Some of you may be having problems logging into Skype," a message on the company's site said. "Our engineering team has determined that it's a software issue. We expect this to be resolved within 12 to 24 hours." The message was posted at 9 a.m. Eastern Thursday. Skype users writing on blogs and message forums, however, have claimed that they have been unable to connect since late Wednesday.

Some users have also said it appears the issue is a log-in authentication failure. "Those that are in stay in and those trying to sign in can't get in," said Tom Keating, who maintains a VoIP blog here.

Although users' Skype contact lists show when the client is launched, most people are finding it impossible to dial out or open an instant message session with any of their contacts. A "Connecting" message at the bottom of the Windows client, for instance, does not go away.

Most of Skype's Web site is accessible, although some sections, particularly the Help and Share areas, are slow to respond and may time out before rendering. Oddly, although the problem acknowledgment message is prominent on the Skype service status page, the indicators claim "All working normally."

Skype was unavailable for comment.

Skype rarely goes offline. The last reported outage resulted in the service going dark for several hours in October 2005.

The company recommended that users leave their client software active. "As soon as the issue is resolved, you will be logged in. We apologize for the inconvenience," Skype said.