Washington ? Federal Reserve Chairman Alan Greenspan urged Congress on Wednesday to restrict the multibillion-dollar holdings of the mortgage companies Fannie Mae and Freddie Mac, warning that their huge debt could imperil U.S. financial markets.

His admonition lent support to an effort in Congress to tighten controls on the two government-sponsored companies following their accounting scandals. Even so, shares of the companies rose in trading Wednesday.

The Fed chairman told the Senate Banking, Housing and Urban Affairs Committee that it might not be enough to just create a strong regulator for the companies, which hold or guarantee more than 45 percent of all mortgage loans in the country.

Legislation re-cently proposed would set up a regulatory agency with the power to compel the companies to sell off any assets deemed not to be in line with their mission of making homeownership more widely available.

The measures would not have Congress set binding limits on the size of the companies’ portfolios, which together have grown to $1.5 trillion. They also have issued $1.8 trillion in debt.

“World-class regulation, by itself, may not be sufficient,” Greenspan testified. “Without restrictions on the size of (their) balance sheets, we put at risk our ability to preserve safe and sound financial markets in the United States, a key ingredient of support for homeownership,” he said.

Portfolio restrictions would not affect mortgage rates for homeowners because so many big banks and other lenders compete with Fannie Mae and Freddie Mac, Greenspan said.