"Our engineers and site operations team are working non-stop to get things back to normal - thanks for your continued patience," Luxembourg-based Skype said in a message on its Twitter feed @Skype.
In a blog post, Skype explained that computers known as "supernodes" had been taken offline by an unspecified problem affecting some versions of Skype.
"Under normal circumstances, there are a large number of supernodes available," Skype said. "Our engineers are creating new 'mega-supernodes' as fast as they can, which should gradually return things to normal.
"This may take a few hours, and we sincerely apologise for the disruption to your conversations," Skype said. "Some features, like group video calling, may take longer to return to normal."
Skype, which was founded in 2003, bypasses the standard telephone network by channelling voice, video and text conversations over the Internet.
The service has millions of users around the world and many took to Twitter to complain about the outage in a variety of languages.