The New York Stock Exchange and NYSE Amex Cash Markets on Thursday invoked a rule to smooth trading at the market open, as futures pointed to a sharp drop in the major indexes.

Rule 48 allows the exchange to suspend price indications that help determine the floor price at the open during regular sessions. Bypassing the requirement helps speed the beginning of trading.

Among the triggers for invoking the rule are substantial activity in the futures market before the open, according to the exchange's website.

U.S. stock index futures pointed to a sharply lower open as a report that regulators were intensifying their review of European banks' U.S. units shook up investors.

S&P 500 futures fell 27.8 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration of the contract. Dow Jones industrial average futures lost 228 points and Nasdaq 100 futures dropped 50 points.

(Reporting by Rodrigo Campos, Editing by Chizu Nomiyama)