Asian investors won’t buy debt and mortgage-backed securities from Fannie Mae and Freddie Mac until they carry explicit U.S. guarantees, similar to those given on bonds issued by Bank of America Corp. or Citigroup Inc.

The risks are too great without a pledge that the U.S. will repay the debt no matter what, according to Hideo Shimomura, chief fund investor in Tokyo for Mitsubishi UFJ Asset Management Co., and other bondholders and analysts in Japan, China and South Korea interviewed by Bloomberg. Overseas resistance may hamper U.S. efforts to hold down home-loan rates and shore up the nation’s largest mortgage-finance companies.

Even after President Barack Obama vowed on Feb. 18 to sink as much as $400 billion of capital into Fannie Mae and Freddie Mac, double the original commitment, “there is still a concern that there is no guarantee” from the government, said Shimomura, who oversees $4 billion in non-yen bonds for the arm of Japan’s largest bank.

“Looking at the risk, they’re not so attractive,” he said. “We need a guarantee before we’ll buy.”

Foreign investors sold $170 billion of agency debt and securities in the second half of 2008, the largest amount since the Treasury began tracking sales in 1977, according to the most recent data. Asians, the biggest non-U.S. block of owners in the category, unloaded $70 billion worth from July through December, after scooping up $55 billion in the second quarter and being net buyers during much of the last decade.

via Bloomberg.com: Exclusive.