Microsoft is acknowledging that a substantial number of its MSN services customers experienced a major service outage on Friday, but is still saying little about the cause.

By Saturday morning, Hotmail, Passport and MSN Messenger seemed to be functioning properly again. Microsoft did not respond by press time with an update as to when the sites came back up or what caused the outage.

Some news sites on the Internet reported that the MSN services were restored by 4:30 p.m. PST on Friday, with Microsoft attributing the outage to an unidentified "internal issue," as opposed to a distributed denial-of-service attack or other malicious event.

On Friday, a Microsoft spokeswoman noted that the first reports of problems accessing MSN Messenger, Hotmail and other MSN-related services, such as Passport, began at 8:30 a.m. PST on Friday.

"While we do not have an exact count regarding the customers affected by this issue, it appears to affect a significant portion of our customers," said an MSN spokeswoman.

"Customers that are already logged into Hotmail, MSN Messenger or other MSN services should be able to continue to use those services," she said. "This issue is primarily affecting new logons and is not affecting all of our customers.

"We're actively investigating the cause and are working to take the appropriate steps to remedy the situation as rapidly as possible," the spokeswoman added. " We sincerely apologize for any inconvenience and disruption this may be causing our customers."

As of 2:45 PST, Microsoft still had no update to its statement that the cause of the outage was unknown.

Roopak Patel, a senior analyst with the public services division of the Internet performance-analysis firm Keynote Systems said that so far Keynote was unable to conclude why Microsoft's various MSN sites were down.

Patel said that Keynote had tried accessing Hotmail, Passport and MSN.com from a number of cities across the country, starting at 1 p.m. PST, and had a 50 percent success rate.

"We found whenever we could get through, performance was very sluggish for the full pages to download," Patel said.

Patel added that Keynote also tried run some trace routes directly to the sites, but "we couldn't even get close to where their edge routers are."

Patel said the MSN problems were not affecting overall Internet performance. And as to what was behind the problems, he noted that "right now, don't have enough data to make any conclusions."

One Hotmail user who was unable to log into his account said "Microsoft should really be much more aware of quality of service issues, particularly for premium Hotmail customers that are paying for the service. The customer said he thought Microsoft should
" have priority access or dedicated servers for those people that depend on the service for e-mail and are paying them."

(Note: This story was updated on Saturday with follow-up information on the status of the Microsoft MSN sites.)