Microsoft has acknowledged a problem that is preventing Hotmail users from accessing their accounts.

"The issue is purely impacting the log-in process for customers and largely does not impact customers who were already logged in," said the company.

"We have made significant progress in decreasing the number of customers currently affected since initial reports, but the issue has not yet been completely resolved. Microsoft is working quickly and aggressively to resolve the issue and expects to restore normal operation to all customers shortly."

Commenters on a Seattle-Post Intelligencer blog from countries around the world, including the Netherlands, Finland, France, Canada and across the US, said they weren't able to access their Hotmail accounts.

Some customers have also reported trouble accessing the Messenger service.

Despite Microsoft's acknowledgement of a problem, its Hotmail network status page reported that the service was still available and there were no known network issues.

Microsoft did not say when the problem started, except to say it began happening early on yesterday. The company said the issue was affecting some Windows Live customers around the world.

Hotmail had an outage of several hours late last year that left some users unable to access their accounts. That issue arose during maintenance work on the service.