Microsoft is investigating an ongoing incident that is preventing some customers from accessing Microsoft 365 applications.

While the company has yet to share which regions are currently affected by this ongoing issue, it has been tagged as an incident in the admin center, a designation typically used for service issues with noticeable user impact.

According to a service alert seen by BleepingComputer, Redmond is currently reviewing telemetry data to discover the root cause and develop a fix.

"Some users may be unable to access Microsoft 365 applications. Impact is specific to some users who are served through the affected infrastructure, attempting to access Microsoft 365 apps," the company said when first acknowledging the outage on Monday morning, at 05:06 AM UTC.

"We're continuing to analyze service telemetry alongside recent changes made to the service, to help identify the root cause and define a clear path to resolution," it added at 9:00 AM, almost four hours into the incident.

This follows two other major incidents that impacted Microsoft 365 last week, blocking customers' access to multiple services and platforms.

On Wednesday, Microsoft mitigated an outage preventing users worldwide from accessing Microsoft Teams, Exchange Online, and the admin center via Microsoft Entra single sign-on (SSO) authentication due to Multi-Factor Authentication (MFA) issues.

One day later, the company resolved another incident that took down some Microsoft 365 services for customers across Europe, Africa, and the Middle East due to an Azure Front Door content delivery network (CDN) issue.

In September, an Exchange Online outage also affected customers worldwide, blocking access to emails and calendars due to a coding bug that caused repeated database dismounts and failovers, leading to increased CPU utilization.

This is a developing story ...