NEW YORK (Reuters) - MBIA Inc, the world's largest bond insurer, on Friday said it had asked Fitch Ratings to withdraw its insurer financial strength ratings for six units, including MBIA Insurance Corp., its main bond insurance arm.

Fitch is the only one of the three major US credit rating agencies that was still reviewing MBIA's critical "AAA" ratings for a possible downgrade. It has also been faster than larger rivals Moody's Investors Service and Standard & Poor's to downgrade other bond insurers' ratings.

MBIA spokesman Willard Hill said Fitch's rating practices were "not at all" a factor in the withdrawal request. MBIA did ask Fitch to continue rating more than $1.1 billion of unsecured debt issued by the company.

MBIA said in a statement that Fitch's rating process "differs in many significant respects from those of the other rating agencies, which affects how investors assess value".

"In turbulent times, the impact of this difference becomes significant, raising the risk of misinterpretation," MBIA added.

Fitch responded on Friday, saying that it was disappointed with MBIA's request as well as the criticism of its analysis and fees. "Our analysis is of the highest quality and our understanding of MBIA's municipal and structured exposure is very strong," Stephen Joynt, president and CEO at Fitch said in a statement.

Fitch added that is unsure whether it will be able to continue to have an insurer financial strength rating for MBIA, as rating the company will be difficult without access to non-public details of their insured portfolio.

Losing "AAA" ratings would make it harder for Armonk, New York-based MBIA to win business, and could further roil credit markets by forcing investors who demand those ratings to sell municipal bonds.

MBIA raised some $2.6 billion of capital to help preserve its "AAA" ratings, after losses mounted from its guarantees of securities tied to subprime mortgages.

Its efforts convinced Moody's and S&P to remove their threats of a downgrade, though they still have a "negative" outlook for MBIA ratings.

MBIA's new chief executive, Joseph (Jay) Brown, said last month he plans within five years to split the company's municipal and structured finance operations.