Tencent said on Monday the company is collecting criminal evidence after a large number of users reported their QQ numbers were stolen and used to send advertisements to their friends on the instant messaging service.

The QQ security team launched an investigation immediately after the company received the feedback from some users at around 10 pm on Sunday, the company said in a statement on its official Weibo account.

The investigation found users had scanned the QR code for a game login forged by the suspects. The login data was then recorded by a criminal gang for illegal use.

After confirming the information, the company organized a full-spectrum security response. At present, the breach has been controlled and affected user accounts have returned to normal in the early morning, it said.

The company is collecting evidence and will cooperate with the authorities to protect the rights of users.

Users shouldn't scan QR codes from unknown sources and be vigilant when logging in to accounts to prevent the risk of theft, it said.