Billionaire Ken Griffin's Citadel will take a tougher stance against the U.S. Securities and Exchange Commission and is willing to take the regulator to court over its WhatsApp probe, Bloomberg News reported on Wednesday citing people familiar with the matter.

The SEC has collected thousands of staff messages from more than a dozen major investment companies, escalating its probe into Wall Street's use of private messaging apps to discuss work, Reuters had reported earlier this week.

The firms targeted by the SEC include Carlyle Group , Apollo Global Management, KKR & Co, TPG, and Blackstone, as well as some hedge funds such as Citadel.

A spokesperson from the SEC declined to comment. Citadel did not immediately respond to a Reuters request for comment.

The Miami-based firm would be the first to take the SEC to court over allegations of untracked communications, the report added, in contrast to almost two dozen banks that have chosen to pay hefty settlements over the last couple of years.

(Reporting by Pritam Biswas in Bengaluru; Editing by Krishna Chandra Eluri)