By Al Yoon

NEW YORK (Reuters) - Central banks around the world bought nearly half of the $8 billion in debt sold by Freddie Mac (FRE.N: Quote, Profile, Research) on Wednesday, potentially bringing their holdings of such debt to record levels.

Central banks purchased $3.88 billion in Freddie Mac two- and five-year debt issues that the company says will be used to finance significant growth of its mortgage portfolio. Demand from the banks may be more crucial in coming months as Freddie Mac and Fannie Mae (FNM.N: Quote, Profile, Research) are pushed to do more to support the housing market, despite their own struggles with losses.

Foreign central banks held $931.9 billion of agency and mortgage-backed securities issued by Fannie Mae, Freddie Mac and other government-sponsored enterprises as of May 14, according to the Federal Reserve. The holdings, just shy of the record $934.8 billion in April, have spiked from $832 billion in December and nearly $600 billion at the end of 2006.

Freddie Mac and Fannie Mae are public companies but hold charters from Congress to support housing by raising money from investors by issuing mortgage-backed securities and selling bonds in the $3 trillion "federal agency" debt market.

Lawmakers faced with rising foreclosures this year are leaning harder on the companies to do more to reverse the housing slump, and have proposed that Fannie Mae and Freddie Mac pay for an expansion in a government mortgage program.

A housing rescue bill passed by the Senate Banking Committee on Tuesday included a provision to have Fannie Mae and Freddie Mac contribute to a fund for the Federal Housing Administration, the agency picking up risky borrowers left in a bind by the credit crunch. Such a link to the government would cement already strong beliefs of implicit U.S. backing, an important assumption for central banks, analysts said.

"It has become clearer and clearer, given the role the GSEs are now playing in stabilizing the mortgage markets, that links to the government are stronger than ever," said Rajiv Setia, a strategist at Barclays Capital in New York.

But the cost to the companies could also hurt shareholders and, to a lesser degree, bondholders, since it may pressure profit, he said in a research note on Monday.