As Skype's outage continues, speculation about its causes is gearing up.

In a posting on its website, Skype blamed the issues on a "deficiency in an algorithm within Skype networking software. This controls the interaction between the user’s own Skype client and the rest of the Skype network."

A later update informed users that the problems will likely continue througout the day on Friday. Users started reporting problems with the free internet calling service on Thursday morning.

Russian security vendor Positive Technologies on Friday offered a different explanation, claiming that it had uncovered a Denial of Service (DoS) exploit that it prevented the service from functioning.

"The program uses a standard Skype client to dial a special number," the firm said in a posting on its website. "That call causes a denial of service on the Skype server, which forwards to a backup server. The new Skype server also crashes, and so on. This way the attack spreads with lightning speed over the entire Skype peer-to-peer network."

Skype denied that it had fallen victim to a denial of service attack.

David Marcus, security research and communications manager with McAfee, told vnunet.com that the firm hadn't seen any spikes in internet traffic that are common for denial of service attacks. He suggested that the Russian message was a hoax.

Skype also denied that the problems were related to a scheduled update that was performed on Wednesday.