Hundreds of additional stocks and 344 exchange-traded funds face trading curbs under a proposed expansion of the circuit breakers that were adopted in response to May's severe flash crash.

The Securities and Exchange Commission earlier this month imposed stock-specific circuit breakers, or a mechanism to pause trading, in all stocks in the Standard & Poor's 500 index.

The regulator on Wednesday proposed expanding the circuit breakers to include all stocks in the Russell 1000 Index and a select group of exchange-traded funds, or ETFs, which were hit harder than ordinary stocks in May's brief market plunge.

The circuit breaker, which halts trading in a stock for five minutes if that stock moves more than 10 percent in five minutes, was launched earlier this month in a pilot program to give the SEC time to make adjustments.

It is my hope to continue to expand the program to additional publicly traded companies, SEC Chairman Mary Schapiro said in a statement.

Regulators have yet to determine what caused the Dow Jones Industrial average to drop some 700 points in a few minutes on May 6 before sharply rebounding.

Exchange operator NYSE Euronext said on Wednesday that U.S. stock exchange operators proposed adding the breakers to 344 exchange-traded products, or ETPs -- which trade like stocks but track an underlying index or basket of assets and have exploded in popularity in recent years.

The proposed list identifies those ETPs that have component securities that largely track the securities included in the S&P 500 and Russell 1000, NYSE Euronext said.

Leveraged exchange-traded funds and other ETPs will be added later, possibly with different trigger rules, the New York Stock Exchange parent added.

The new breakers kicked in at least twice in the last few weeks, most recently when an erroneous trade in Citigroup Inc briefly halted the bank's shares on Tuesday.

The SEC expansion proposal will be open for a 10-day public comment period after it has been published in the government's official register. The SEC still has to adopt the rule in order for it to go into effect.

(Reporting by Rachelle Younglai, additional reporting by Jonathan Spicer in New York; editing by Andre Grenon, Phil Berlowitz)

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