NEW YORK, (Reuters) - Ambac Financial Group Inc , the second-largest US bond insurer, posted a $2.34 billion fourth-quarter loss yesterday, as it set aside more money for troubled mortgage debt.

The company also said it ended the year with a negative net worth of $3.78 billion, reflecting the declining market value of securities tied to home loans, which it guaranteed before the nation's housing slump began.

Ambac and larger rival MBIA Inc made a disastrous move into structured finance earlier this decade in a bid to boost profits, and diversify from the relatively safe but lower-margin business of guaranteeing municipal bond payments.

"We have learned from this, and will not stray again," Ambac chief executive David Wallis said on a conference call.

Ambac said it was continuing with plans to start a new municipal bond insurer, Everspan Financial Guarantee Corp, a week after MBIA set plans to split its municipal bond business from its own troubled structured finance operations.

Ambac's fourth-quarter loss was $8.14 per share, compared with a year-earlier loss of $32.03 per share, or a net loss of $3.27 billion.

The latest results included an increase of $1.53 billion in the amount set aside because it might be unable to use some tax credits. Year-earlier results included big writedowns for credit derivatives linked to assets such as mortgages.

Excluding securities gains, the quarterly operating loss was $6.79 per share. Analysts, on average, expected a loss of 68 cents per share, according to Reuters Estimates. It was not immediately known if the two numbers were comparable.

Ambac expects its new municipal insurer, Everspan, to start doing business in the second quarter.