Calpers, the biggest U.S. public pension fund, has filed a suit in a California state court in connection with $1 billion in losses that it says were caused by inaccurate credit ratings from the three leading ratings agencies, The New York Times reported.

The lawsuit, filed late last week in California Superior Court in San Francisco, is focused on a form of debt called structured investment vehicles, highly complex packages of securities made up of a variety of assets, including subprime mortgages, the paper said.

Calpers bought $1.3 billion of them in 2006; they collapsed in 2007 and 2008, the paper added.

Calpers maintains that in giving these packages of securities the agencies' highest credit rating, the three top ratings agencies -- Moody's Investors Service, Standard & Poor's and Fitch -- "made negligent misrepresentation" to the pension fund, the daily said.

The AAA ratings given by the agencies "proved to be wildly inaccurate and unreasonably high," the paper said, citing the suit.

According to the paper, Calpers is seeking damages, but did not specify an amount.

Calpers, Moody's, Standard & Poor's, and Fitch could not be immediately reached for comment by Reuters.