Ambac estimates Jan derivatives losses at $650 mln

NEW YORK (Reuters) - Ambac Financial Group Inc (ABK.N) said on Friday that the market value of its credit derivatives fell about $650 million in January, and declines in value continued into February.

Ambac wrote down the value of its credit derivatives contracts by $6 billion in 2007, creating a net loss of $3.248 billion for the year.

Ambac also said that as assets it has guaranteed using credit derivatives are downgraded, it may have to raise additional capital to support its exposure, or limit its writing of new business.

(Reporting by Dan Wilchins; editing by Carol Bishopric)