Some Gmail users have claimed that they have been receiving ads sent to them from their own accounts despite updated passwords and two-factor authentication, the media reported.
To get the messages past spam filters, the spammers have been using forged email headers to make them appear as though they are being sent via a "Canadian telecommunications company" called "Telus", Mashable reported on Sunday.
The subject of the emails read "weight loss and growth supplements for men".
"We are aware of a spam campaign impacting a small subset of Gmail users and have actively taken measures to protect against it," a Google spokesperson was quoted as saying.
"We have identified and are reclassifying all offending emails as spam, and have no reason to believe any accounts were compromised as part of this incident," he added.
Because the messages appeared to be coming from the same user, Gmail filed the message into affected users' sent folder.
"My email account has sent out 3 spam emails in the past hour to a list of about 10 addresses that I don't recongnise. I changed my password immediately after the first one, but then it happened again 2 more times," a user wrote on "Google Help Forum".
"The company's engineering teams are aware of this and are working on a resolution," Google employee Seth Vargo tweeted in reply to one such complaint.