Fannie Mae (FNM) saw the largest increase in a month of its single-family delinquency rate among prime borrowers in January.

The mortgage finance company said this rate shot up to a historic high of 2.77% in January from 2.42% in December, a record 35 basis point increase that hasn't been seen since the company started tracking these numbers in 1998. This compares to a delinquency rate of 1.06% in January 2008.

While these numbers are still low, it represents an increased stress on the company from its mortgage holdings. These numbers are expected to rise as the unemployment rate continues to tick upwards, and job losses drag even credit-worthy borrowers into missing payments.

On the business front, Fannie committed to buy nearly $2.4 billion of mortgage bonds in February, down a bit from its net commitments of nearly $3 billion in January.

The mortgage giant's investment portfolio shrunk by 1.3%, keeping its total balance at $784.7 billion, well short of the curbs set by its regulator, according to a monthly report from the company.

Over the past couple of months, the role of Freddie Mac (FRE) and its sibling Fannie in the mortgage market have diminished as both the U.S. Treasury and the Federal Reserve have emerged as backstop buyers with deep pockets.

The U.S. Treasury, so far, has bought nearly $107 billion of agency mortgage-backed securities, while the central bank bought nearly $270 billion.

However, market participants still keep tabs on Fannie and Freddie's portfolios as an indication of their financial health, and their ability to continue to play a role as both guarantors and buyers of mortgage bonds.

Meanwhile, Fannie Mae's total book of business increased at an annualized compound rate of 2% in February.

Total Fannie Mae issuance of mortgage bonds increased to $45.3 billion in January, more than double its January volume of $21.2 billion.

Issuance of Fannie Mae securities and other guarantees increased at a compounded annualized rate of 1.3% during the month.

Fannie's duration gap, a measure of the portfolio's sensitivity to interest rates, averaged three months in February, up from two in January.

Freddie and Fannie are chartered by Congress to buy mortgages from lenders, freeing them to make more loans.

They repackage the mortgages as securities and sell them again. Both also hold on to large quantities of mortgage securities, profiting from the difference between the interest rates they pay and the cost of debt issued to fund their purchases.

-By Prabha Natarajan, Dow Jones Newswires; 201-938-5071; prabha.natarajan@dowjones.com