Skype's Internet-calling service "has not crashed or been victim of a cyber attack," the eBay subsidiary has claimed, after some users were unable to log on for hours.

The company said the problem was caused by "a deficiency in an algorithm in Skype networking software" in a blog posting at 10pm GMT yesterday. It had earlier fingered an unspecified software problem. Skype was working to fix the problem, the company said.

At about 2am GMT, Skype posted an acknowledgement of the log-in problem and said it expected to solve the problem within 12 to 24 hours. The latest update was posted about 20 hours later. Downloads of the Skype software had been disabled at that time.

Skype's peer-to-peer software enables Internet voice calls, messages and video. It's free to use among software clients on PCs and hand-helds, and Skype also offers a low-cost service for reaching conventional phones.

At the end of June, the service had 220 million registered user accounts, up 94 percent from a year earlier. Skype recently has been trying to address security concerns to appeal to enterprises.