SAN FRANCISCO—Twitter says it mistakenly reset the passwords of some users as part of a routine security check-up.

Thursday's mix-up triggered warnings on technology blogs and in tweets that the online messaging service had been attacked by computer hackers.

Twitter sought to clear up the confusion with a post on its website a few hours after it sent out emails alerting some users that their accounts had been compromised. Twitter says its security specialists had reset a larger number of passwords than they intended.

Despite the mistake, Twitter says only a very small percentage of its more than 140 million users had their passwords affected.

Twitter, which is based in San Francisco, regularly resets the passwords of accounts believed to have been infiltrated by hackers or suspicious websites.