UPDATE 1-Fannie Mae mortgage holdings, delinquencies climb

(Adds details, Freddie Mac portfolio)

NEW YORK, July 30 (Reuters) - Fannie Mae FNM.N FNM.P said its gross mortgage portfolio rose by a 4.6 percent annual rate in June, following a 35 percent jump in May, while the serious delinquency rate on loans it guarantees kept climbing.

The largest U.S. home funding company's mortgage holdings grew to $792.6 billion, up just 1.4 percent year-to-date and well below the maximum $900 billion allowed by its regulator.

After the government took over the capital-strained company last September, it said Fannie Mae as well as Freddie Mac FRE.N FRE.P could increase their holdings to that cap this year before starting to reduce them next year.

The rate of serious delinquent payments on single-family mortgages that Fannie Mae guarantees rose 26 basis points to 3.68 percent in May, the latest data available. A year earlier, the rate was 1.30 percent.

On the multifamily side of the business, the serious delinquency rate rose 14 basis points to 0.50 percent, up sharply from 0.09 percent a year earlier.

Fannie Mae said it securitized $33.2 billion of whole loans in June that it is holding for investment in its own portfolio. In May, it securitized $61.4 billion of these loans.

The company's total book of business, which includes the assets in portfolio as well as Fannie Mae mortgage bonds and other guarantees, expanded by an annual 18.1 percent in June to $3.19 trillion. That put year-to-date growth an an annual 5.4 percent rate.

Last week, Freddie Mac reported that its investment portfolio rose by an annual 9.3 percent rate in June to $829.8 billion. The rate of serious delinquency on the single-family loans it guarantees also continued to increase. (Reporting by Lynn Adler; Editing by James Dalgleish)