UBS AG is under pressure for failing to monitor an equity trader suspected of causing a $2.3 billion loss for the Swiss bank, but its exposure to U.S. class-action litigation over the scandal may be limited.

Following negative announcements from companies, lawyers for U.S. shareholders routinely file class-actions in American courts alleging that misrepresentations were made to investors. But so far, plaintiffs lawyers who specialize in these kinds of cases have been quiet in response to the UBS scandal.

As of Monday afternoon, there had been no public announcements of any securities fraud lawsuits filed over the UBS trading losses, although some lawyers said they are keeping their options open.

We'll look closely at it, said Steven Toll, a prominent class-action lawyer from law firm Cohen Milstein Sellers & Toll in Washington, D.C.

Last Thursday, UBS stunned markets when it announced that a London trader concealed unauthorized speculative trading in various S&P 500, DAX and EuroStoxx index futures over the last three months by creating fictitious hedging positions in internal systems. Trader Kweku Adoboli was charged on Friday with fraud and false accounting dating to 2008.

It was the latest embarrassment for UBS, which has sought to recover from its near collapse during the financial crisis and an investigation into allegations it helped wealthy U.S. clients evade paying taxes.

But the scandal will present obstacles to investors who try to seek redress in U.S. courts.

Shareholder damages appear relatively small. On the day before UBS announced its trading losses, the company's shares on the New York Stock Exchange closed at $12.68. The next day they closed at $11.41.

Under U.S. securities laws, damages cannot exceed the difference between the price investors paid for their shares and the average price of the stock over a 90-day period beginning on the date when the information correcting the misstatement was disseminated.

If, during 90-day period, shares of UBS rise significantly, it could negate any potential damages.

Another obstacle for plaintiffs is a U.S. Supreme Court ruling that recently stopped a case against Societe Generale over the company's failure to stop unauthorized trades by employee Jerome Kerviel, who racked up a $6.7 billion loss in 2008. Kerviel was sentenced to three years in prison in October 2010 by a Paris court.

In a lawsuit brought in Manhattan federal court, shareholders alleged Societe Generale ignored internal red flags of Kerviel's trading and misrepresented the quality of its management policies and internal controls to investors.

As a result, Societe Generale's corporate and investment banking division was forced to restate 98 percent of its originally reported net income for the six-month period ended June 30, 2007, according to the plaintiffs.

But in dismissing the case, U.S. District Judge Richard Berman in Manhattan last September cited the U.S. Supreme Court's 2010 ruling, Morrison v. National Australia Bank Ltd, which barred shareholders from bringing claims for shares purchased on foreign exchanges.

Berman found that because the plaintiffs -- Vermont Pension Investment Committee and Boilermaker-Blacksmith National Pension Fund -- had purchased their Societe Generale shares on foreign exchanges, they were barred from bringing claims in the United States.

UBS itself was the beneficiary of the Morrison ruling just last week in a case alleging the bank misled investors about its exposure to subprime mortgages and its allegedly illegal sheltering of U.S. tax evaders.

The plaintiffs, who claimed more than $100 billion in losses, attempted a novel argument to avoid the Supreme Court's Morrison ruling. They argued that, while their shares were purchased abroad, they should be able to bring claims in the United States because UBS common shares are traded on the New York Stock Exchange.

In a decision on Tuesday, U.S. District Court Judge Richard Sullivan in Manhattan rejected that argument and in the process wiped out almost 90 percent of UBS's exposure in the lawsuit.

Because only about 12 percent of UBS shares are listed in the United States, the bank's exposure would be limited if shareholders brought a class-action case over the trading scandal, said Hannah Buxbaum, a professor at Indiana University's law school.

It's a relatively small number of plaintiffs' claims that would survive a Morrison review, she said.

(Reporting by Andrew Longstreth in New York; editing by Martha Graybow and Andre Grenon)