NEW YORK — Yahoo said Thursday usernames and passwords of its e-mail customers have been stolen and used to access accounts, but the company isn’t saying how many accounts have been affected.

Yahoo is the second-largest e-mail service worldwide, after Google’s Gmail, according to the research firm comScore. There are 273 million Yahoo mail accounts worldwide, including 81 million in the U.S.

Yahoo said in a blog post on its breach that “The information sought in the attack seems to be names and e-mail addresses from the affected accounts’ most recent sent e-mails.”