NEW YORK, May 7 (Reuters) - MBIA (MBI.N: Quote, Profile, Research), the world's largest bond insurer, sued two of its own insurers on Wednesday, accusing them of refusing to pay for defense costs stemming from regulators' probes.

MBIA wants the insurers to cover its "total defense costs" from investigations by the U.S. Securities and Exchange Commission and the New York State Attorney General, as well as from a shareholders' lawsuit and separate class-action lawsuit, according to the lawsuit, filed in the U.S. District Court in Manhattan.

In January 2007, MBIA agreed to settle the probes with the SEC and the New York State Attorney General for $75 million and to name an independent consultant to review its accounting procedures.

MBIA had Directors and Officers liability insurance policies with Federal Insurance Company, a unit of Chubb Corp (CB.N: Quote, Profile, Research), and ACE American Insurance Company, a unit of Ace LTD (ACE.N: Quote, Profile, Research), the lawsuit said.

The policy with Federal Insurance was to cover the first $15 million of defense costs, subject to a $2.5 million deductible; and the one with ACE was to cover the next $15 million, the lawsuit said.

MBIA defense costs have "greatly exceeded the $15 million limit" it said in the lawsuit, but it said it has received $6.4 million from Federal and nothing from Ace.

Calls to Federal and ACE for comment were not returned.

The lawsuit said the two insurers were also withholding their approval of an offer to settle a class-action lawsuit and were refusing to pay defense costs from two derivative shareholder suits that have been dismissed. (Reporting by Leslie Gevirtz; Editing by Toni Reinhold)