Microsoft says it is working to tackle "connectivity problems" after users worldwide reported outages on Thursday.
Services including Teams, Outlook and Microsoft's AI software Copilot were affected.
The tech giant said a part of its infrastructure was experiencing "elevated load", which was likely contributing to connectivity errors.
While the issue was originally reported in the US, Microsoft said the connectivity problems could affect users "in any region".
At around 18:30 GMT on Thursday evening, monitoring website Downdetector showed a spike in UK users reporting issues with Microsoft products.
Similarly for the US, the monitoring site suggested there had been over 131,000 reports of Microsoft Outlook problems at 18:15 GMT.
"We determined the elevated load isn't driven by an increase in traffic, but by a number of servers being unexpectedly unavailable; straining the remaining infrastructure," Microsoft said in a statement.
In a further update issued at 19:50 GMT, Microsoft said it was "testing a configuration" to fix the problem, which was "causing servers to fail automated health checks and become unavailable".
All affected servers had been restarted, it added.
