MBIA Inc., the world's largest bond insurer, said it has $9 billion of exposure to the riskiest mortgage debt, $900 million more than the company disclosed about three weeks ago.

The company, in a public filing on Jan. 9, said it had guaranteed $30.6 billion of complex securities known as collateralized debt obligations. Of those securities, MBIA said it has $9 billion of exposure to the riskiest structures known as CDOs of CDOs, or CDO squared.

About 70 percent of the collateral of the CDO squared structures are rated triple-A, the highest grade. MBIA said 41 percent of the securities were originated in 2005 or before.

Last month the company said it insured $8.1 billion of CDO squared debt, surprising investors and sparking the biggest one-day decline of MBIA's shares.

MBIA shares on Friday rose 18.1 percent to $16.66.

MBIA also said it now has $45.2 billion of exposure to overall residential mortgage-backed securities, which comprises 7 percent of MBIA's insured portfolio, as of Sept. 30, 2007.

Spokespersons for MBIA didn't return phone calls seeking comment. (Editing by Leslie Adler)