Skype is currently down for millions of users, according to reports.
The company said that some users were encountering problems in a Twitter post on Wednesday afternoon.
"Some of you may have problems signing in to Skype: we're investigating, and we're sorry for the disruption to your conversations," the tweet from the official Skype Twitter account said.
The number of online users on the internet telephony service fell from 21 million to 12.5 million in 30 minutes, according to post on ReadWriteWeb.
As of 5:17pm, Skype was reporting problems on its text message (Skype SMS) service on its Skype Heartbeat site, which displays the status of Skype services. Heartbeat reported that all other Skype services were "working normally".
Skype's communications network is suffering downtime due to failures in its "supernodes", which sometimes help Skype applications find and establish lines of communication with other computers and phones, Skype said in a blog post on Wednesday evening.
Skype's communications networks relies on "millions of individual connections between computers and phones to keep things up and running", but when a Skype application has trouble connecting to another computer or phone it will turn to a supernode to find the addressing details of the desired connection.
"Unfortunately, today many of [the supernodes] were taken offline by a problem affecting some versions of Skype. As Skype relies on being able to maintain contact with supernodes, it may appear offline for some of you," Skype wrote in the blog post.
Skype is working to fix the problem by creating what it terms "mega-supernodes" to help bring the communications network back online, Skype wrote.
"Our engineers and site operations team are working non-stop to get things back to normal — thanks for your continued patience," Skype also said in a tweet.
Update 12:34pm Thursday
The Skype outage is continuing to affect enterprise users.
Update 11:45am Friday
Skype service has resumed after the addition of the vaunted 'mega-supernodes' stabilised the network.