A hacking group is claiming responsibility for connectivity problems affecting Skype over the last two days.
The Skype outage began on Monday 19 June at 19:01 GMT.
There have been numerous complaints that people were unable to log in, receive messages or make voice calls.
Hacking group CyberTeam announced on Twitter that it was responsible for the attack.
The Skype outage has affected multiple countries across Europe, as well as Japan, Singapore, India, Pakistan and South Africa, according to service monitor Down Detector.
Microsoft has published a blog about the outage but has declined to comment further.
"We are aware of an incident where users will either lose connectivity to the application or may be unable to send or receive messages. Some users will be unable to see a black bar that indicates that a group call is ongoing, and longer delays in adding users to their buddy list," Jagadish Harihara wrote on the blog on Monday.
At 20:00 GMT on Tuesday, he updated the post, saying: "We have made some configuration corrections and mitigated the impact. We are continuing to monitor and we will post an update when the issue is fully resolved."
CyberTeam claimed responsibility for the attack in a tweet, which reads "Skype Down by Cyberteam". A second tweet from the group indicates that it next wants to target the digital gaming platform Steam.
There has been speculation that the Skype outage was caused by a Distributed Denial of Service (DDoS) attack on its network.
"DDoS attacks are some of the most common approaches for taking down networks. Skype have not publicly gone into detail about what is happening, but the disruption we're seeing could certainly be explained by a DDoS attack amongst other possibilities," Dr Steven Murdoch, a cyber-security researcher in the department of computer science at University College London told the Eyes On Events News website.
"Some DDoS attacks can be extremely large and can disrupt even the largest companies. We'll have to wait and see whether Skype could have handled the situation better. But on the upside, if it is a DDoS attack, this does not affect customers' private details."