Google on Tuesday apologized for an outage that struck possibly millions of users of its email system, Gmail, earlier in the day. The scope of the outage was not immediately clear, but at least some users in Europe and Asia could not get access to their inboxes or had to wait a minute or more for them to open.

"We're aware of a problem with Gmail affecting a number of users," Google said in an advisory on its Gmail support site. "We're working hard to resolve this problem and will post updates as we have them. We apologize for any inconvenience that this has caused."

Its email service went offline 10:25 a.m. GMT and affected 113 million users from Europe, Asia, Australia and the U.S. Google claims the downtime was only for two-and-a-half hours. However, users from different parts of the world reported longer offline time running for up to four hours.

However, only web-based Gmail account holders were affected, while those who access their Gmail accounts through mobile phone or third-party mail clients continued to enjoy the service.

A later blog update said that service had been restored, that engineers were investigating the problem and that certain users would have to take an extra step before being able to access their mail.

Google promises that customers paying for the Google Apps service will have access to Gmail at least 99.9 percent of the time each month or Google has to pay a penalty. So far Google hasn't dipped below that, the company said last year. This isn’t Gmail’s first service outage. In August 2008, both Google Gmail and Google Apps experienced nearly 15 hours of downtime, an event that had users flocking to message boards to express their ire.