Users of Skype, Microsoft’s Internet calling unit, faced a lengthy blackout on Monday after people worldwide were not able to make calls or sign into the service.

Customers in Europe, the United States and Japan, starting early morning European time, were affected, according to reports on social media. Skype did not specify how many of its roughly 300 million global users could not use the service.

In a statement, the company said an undisclosed technical issue had caused people’s online contacts to be shown as offline, even when they were signed into Skype. The error meant that some users could not call one another, though people could still send written online messages to their Skype contacts.

The company added that other users were also having problems signing in to the service.

Skype has lately come under increasing pressure from competitors like WhatsApp and Viber, which also allow people to send messages and call one another through a smartphone app.

Later on Monday, more than 12 hours after the error was first detected, Skype said it had identified the problem and was working to fix it.

“We’re in the process of reconnecting our users, and focused on restoring full service,” the company said in a statement.