"Yahoo Groups has been inaccessible due to hardware issues," confirmed Yahoo spokeswoman Mary Osako. "We have identified the problem and we're working aggressively to fix it."
Osako declined to say more about the cause or provide an estimate of when Yahoo expects service to be resumed.
She added: "We know this may have caused some inconvenience, and we apologise to Yahoo Groups."
Users visiting the site at groups.yahoo.com were redirected to a page with a message that said the site was down for maintenance.
"All e-mail sent during this time should be delivered once service has resumed, but further delays may occur due to backlog. Please do not resend e-mail to your group," the message read.
The outage appeared to affect Yahoo Groups worldwide.