The operators of China National Knowledge Infrastructure (CNKI), a leading online academic database, have been penalized for illegal handling of users' personal information, the Cyberspace Administration of China (CAC) announced Wednesday.

A fine of 50 million yuan (about 6.95 million U.S. dollars) has been imposed on three major operators of CNKI, the CAC said in a statement.

The statement said the operators collected users' personal information without consent on the 14 CNKI-related apps and illegally kept their information after they closed their accounts.

The operators were ordered to stop all illegal activities.

The CAC pledged to step up law enforcement concerning cyber security, data security, and personal information protection to create a healthy and well-regulated environment for business operations and protect citizens' legal rights and interests.