(Adds quote, euro government bonds at record levels)

By Walden Siew

NEW YORK, Jan 13 (Reuters) - Standard & Poor's on Tuesday affirmed its "AAA" rating for the United States but said risks to the country's top sovereign grade have increased since the financial crisis deepened in September.

The ballooning costs of rescuing U.S. banks and auto companies, combined with an expected near-$1 trillion stimulus plan by President-elect Barack Obama, "will lead to noticeable deterioration in the U.S. fiscal profile," S&P said.

The Obama administration needs to "flesh out a medium-term strategy," David Beers, S&P's global head of sovereign ratings, said on a conference call. "There are a lot of unanswered questions."

The action was part of a broader review of 20 highly rated sovereign nations that resulted in affirmations for 12 "AAA" nations, including Australia, Austria, Canada, Denmark, Finland, France, Germany, Netherlands, Sweden, Switzerland and the United Kingdom.

S&P also affirmed its ratings for Belgium, Japan and Italy, while saying downgrades of investment-grade nations may be possible for New Zealand, Ireland, Spain, Portugal and Greece.

Rating firms are under fire for having given high ratings to home loans and complex bond packages that later plummeted in value and led to the collapse of the U.S. subprime mortgage market and so to the global financial crisis.

S&P, Moody's Investors Service and Fitch Ratings were "so wrong with subprime and structured finance," said Edward Grebeck, chief executive officer at Tempus Advisors in Stamford, Connecticut. "It's possible it's playing out with sovereigns now."