Bill Gross' Pimco Total Return Fund, the world's largest bond fund, increased its holdings of U.S. government-related debt to 45 percent in December from 37 percent in November, data from the firm's website showed on Friday.

The fund, which has $237 billion in assets, posted an increase in its mortgage holdings to 35 percent in December from 34 percent in November.

The fund showed a negative position of 6 percent in money market and net cash equivalents in December after showing a 5 percent exposure in November. The fund also increased its holdings of non-U.S. developed market securities to 6 percent in December from 4 percent in November.

Pacific Investment Management Co., a unit of European financial services company Allianz SE, had $1.97 trillion in assets as of Sept. 30, 2013, according to the Pimco website.