Bill Gross, manager of top bond fund PIMCO, said on Monday it was a misconception that the firm was betting against U.S. Treasuries, despite his concerns about the U.S. fiscal outlook.

We, in fact, were never short Treasuries, we're very underweight Treasuries, let's put it that way, Gross, the co-chief investment officer of PIMCO, said in an interview with CNBC.

He said PIMCO is outperforming 77 percent of the bond market universe even as Treasuries rally because we hold other bonds that are doing better than Treasuries.

The company's website in May showed PIMCO's $240 billion Total Return fund

was short U.S. government-related debt -- which includes Treasuries, TIPS, agencies, interest rate swaps, Treasury futures and options, and FDIC-guaranteed corporate securities.

Asked by Reuters about which of those assets PIMCO was shorting rather than Treasuries, a spokesperson said the firm does not break down that information.

PIMCO started betting against U.S. government-related debt in April, with a short position equivalent to 3 percent of the assets in its Total Return fund.

The fund increased that short position to 4 percent this month as the Federal Reserve's bond purchase program neared its scheduled end in June, raising worries as to who will support the government bond market after that.

(Editing by Padraic Cassidy and Leslie Adler)