OMAHA, Neb. — Billionaire investor Warren Buffett said he offered to shore up $800 billion of municipal bonds guaranteed by troubled MBIA Inc., Ambac Financial Group Inc. and FGIC Corp. in a bid to gain 33 percent of the debt insurance market.

But two of the three troubled bond insurers he offered to help have rejected his offer of extra guarantees on municipal bonds.

Ambac Financial Group Inc. said in a statement Tuesday that the reinsurance that Omaha-based Berkshire Hathaway Inc. is offering wouldn't be in the best interests of the bond insurer or all of its policyholders.

Buffett said one of the three bond insurers Berkshire offered reinsurance to had already said no.