NEW YORK (Reuters) - Assured Guaranty Ltd. shares have been hammered amid the credit crunch, but underwriting business at the Bermuda-based bond insurer was "very good" at the end of last year, billionaire investor and Assured director Wilbur Ross told Reuters.

Despite Moody's Investors Service cutting Assured's credit rating to double-A last month, Assured's primary business of underwriting municipal bonds was proceeding "at a very good clip", Ross said.

"The business had a very good December and fourth quarter now that the ratings picture has stabilised."

The market for bond insurance has remained weak, putting pressure on the credit ratings of companies that provide assurance to muni bond investors.

Assured Guaranty remains triple-A at Standard & Poor's and Fitch Ratings, Ross noted, which is critical since a broad downgrade could shut Assured out of the muni business.

"Now that the credit ratings are stable, Assured is able to function very well in the marketplace," Ross said in an interview. "I'm very happy with the business that Assured has been making. The stock, obviously, has been a disappointment."

Shares of Assured have fallen 60 percent since Ross first announced his intention to purchase a minority stake.