(Recasts, adds Credit Suisse comment)

By Al Yoon

NEW YORK, June 25 (Reuters) - Fannie Mae (FNM.N: Quote, Profile, Research) and Freddie Mac (FRE.N: Quote, Profile, Research), the largest providers of funding for U.S. residential mortgages, on Wednesday said they added to their investments at a swift pace in May, though signs of future growth were uncertain.

Fannie Mae, the larger of the two government-sponsored enterprises, said its portfolio grew last month at a 15 percent annualized rate to $736.9 billion, the highest balance since August 2005.

Freddie Mac's portfolio balance soared at a 53.4 percent rate to a record $770.4 billion.

Shares of Fannie Mae and Freddie Mac jumped about 3 percent and 4 percent, respectively, in New York trading.

Net contracts to purchase mortgages at Fannie Mae declined slightly from April to $45.4 billion. Nearly $21 billion of that was marked as commitments to sell, possibly in response to a recent rally in mortgage bonds, said Jim Vogel, a strategist at FTN Financial Capital Markets in Memphis, Tennessee. But actual portfolio sales have been low, he noted.

At Freddie Mac, mortgage purchase agreements fell 40 percent to $26.2 billion.

The GSEs have sharply increased purchases of mortgages since March after their federal regulator eased requirements on capital they must hold and urged the companies to do more to help stabilize the U.S. housing market. Freddie Mac has yet to raise $5.5 billion in additional capital as promised in May, and a delay could cap its ability to follow through on its intentions to support housing, analysts said.