Ambac sues Bank of America on sour mortgages

* 'Massive fraud' alleged over mortgage securitizations

* Countrywide allegedly misleading over loans, guidelines

* Bank of America had no comment

* Bank of America shares down 0.6 pct; Ambac falls 1.5 pct

NEW YORK, Sept 29 (Reuters) - Ambac Financial Group Inc ABK.N sued Bank of America Corp (BAC.N), alleging a "massive fraud" that caused it several hundred million dollars of losses from insuring mortgage securities that went sour.

In a complaint filed in the New York State Supreme Court in Manhattan, Ambac said the bank's Countrywide mortgage unit had misled it about its loan quality and underwriting guidelines when preparing 12 residential mortgage-backed securitizations between 2004 and 2006.

Bank of America bought Countrywide in July 2008. A spokeswoman, Shirley Norton, said the bank had no comment.

Based in New York, Ambac had been the second-largest U.S. bond insurer before losses on risky debt, including mortgages, caused it in 2008 to lose the "triple-A" credit ratings it depended on to insure bonds, mostly municipal debt.

Shares of Bank of America were down 0.6 percent at $13.19 in afternoon trading, while Ambac fell 1.5 percent to about 56 cents.

The case is Ambac Assurance Corp et al v. Countrywide Home Loans Inc et al, New York State Supreme Court, New York County, No. 651612/2010. (Reporting by Jonathan Stempel; Editing by Lisa Von Ahn)