Money-market mutual funds, in an effort to increase liquidity while reducing market-volatility risk and meeting regulatory requirements, have focused since 2007 on boosting holdings of government debt, according to a report by Fitch Ratings.

U.S. government securities, including Treasurys and agency debt, saw the largest increase among rises in liquid assets held by money funds, according to Fitch’s analysis of data from the 10 largest U.S. prime money-market mutual funds in a report published this past week.

The percentage of fund assets invested in Treasurys and agencies rose to 22 percent as of the end of September, from 2 percent in August 2007, the report said.

Liquid securities overall rose to 45 percent of funds assets at the end of September from about 20 percent at the end of 2006, the Fitch report showed.

The Securities and Exchange Commission enacted several rules changes for money funds in 2010 amid efforts by regulators to make the industry more stable after the Sept. 16, 2008, collapse of the $62.5 billion Reserve Primary Fund.