Goldman Sachs & Co analysts downgraded Citigroup to sell and said the largest U.S. bank may have to write off $15 billion for debt losses over the next two quarters, and it placed it on Americas Sell List.

The report by analysts led by William Tanona came shortly after Citigroup's own chief U.S. equity strategist, Tobias Levkovich, upgraded the nation's banking sector to overweight from market weight, calling selling pressure overdone.

Goldman's projection for a Citigroup write-off compares with the $8 billion to $11 billion that New York-based Citigroup on November 4 said it may need to write off this quarter for a $43 billion CDO exposure tied to subprime mortgages.

The Goldman analysts also cut Citigroup's profit-per-share forecast to $3.80 from $4.65 for 2008, and to $4.60 from $5.20 in 2009. Their 12-month price target is $33 per share.

With deteriorating consumer and housing metrics, Citigroup is facing mounting pressure across many businesses, Tanona wrote. Further magnifying the earnings decline is a peer-low tier-1 capital ratio, which may lead the firm to pursue either a capital infusion or the prospect of reducing its current dividend.

Citigroup shares fell 81 cents, or 2.4 percent, to $33.19 in premarket trading.

Goldman also said Citigroup may struggle to recover, hurt by the absence of a permanent chief executive following Charles Prince's resignation, also on November 4.

(The) risk taking culture may be irreparably damaged, the analysts wrote. Further, the absence of a CEO to direct strategy and risk taking directives may limit trading activities and Citigroup may be unable to either capitalize on business relationships or utilize its balance sheet.

Banks have announced more than $50 billion of write-downs tied to the U.S. housing slump, as defaults soared and the value of mortgages that investors deemed too risky plummeted.

Citigroup has said the $8 billion to $11 billion write-down might grow. It is on top of a $1.83 billion mortgage-related loss that Citigroup took in the third quarter.

The bank's tier-1 capital ratio is 7.32 percent, above the 6 percent at which regulators deem a bank well-capitalized. Its $2.16 per share annual dividend equates to a 6.4 percent dividend yield.

Goldman analysts issued their report after Citigroup strategist Levkovich on November 16 upgraded the U.S. banking sector, saying it has compelling valuation, depressed earnings revision data and awful investor sentiment.

Levkovich also wrote that worries about subprime exposures have created a pile-on effect that seems to be overdone. Levkovich said his upgrade may seem fairly controversial.

(Reporting by Jonathan Stempel; Editing by Steve Orlofsky and Maureen Bavdek)