Google said Thursday that its document writing tool Google Documents was secure even as Russian internet users discovered scores of files that appeared to be intended for private use.

The Russian internet company Yandex said in a statement that some users contacted the company Wednesday to say that its public search engine was yielding what looked like personal Google Documents files, suggesting there may have been a data breach.

On Wednesday night, Russian social media users started posting scores of such documents, including an internal memo from a Russian bank, press summaries and company business plans. The veracity of those documents could not be independently confirmed.

Google said in a statement that search engines can only turn up Google documents that had either been deliberately made public by its authors or when a user publishes a link to a document and makes it available for public access and search.

"Saving and protecting users' personal data is our main priority," the company said. "Google Documents is a highly protected tool for joint work and it is working correctly."

It did not elaborate.

Ilya Grabovsky, a spokesman for Yandex, said its search only yields files that don't require logins or passwords. He added that the files were also turning up in other search engines.