Yahoo's Head of UK Consumer PR Caroline MacLeod-Smith said that she couldn't immediately provide any more detail on the breach "as we are still investigating it."
Technology news websites including CNET, Ars Technica, and Mashable cited hackers calling themselves the D33D Company as claiming responsibility for the attack, adding that data posted to the group's website carried more than 453,000 login credentials from an unidentified Yahoo subdomain.
The little-known group was quoted as saying that they had stolen the passwords using an SQL injection - the name given to a commonly-used attack in which hackers use rogue commands to extract data from vulnerable websites.
"We hope that the parties responsible for managing the security of this subdomain will take this as a wake-up call," the group was quoted as saying.
A Ukraine-registered website associated with D33D Company appeared to be unreachable Thursday an email address and a phone number attributed to the site's registrant appeared to be invalid.