More stocks and certain exchange traded funds would face trading curbs when markets are in free-fall, the Securities and Exchange Commission proposed on Wednesday.

In response to May's unexplained flash crash, the SEC imposed circuit breakers, or a mechanism to pause trading, in all stocks in the Standard & Poor's 500 index.

The regulator has now proposed expanding circuit breakers to include all stocks in the Russell 1000 Index and a select group of exchange-traded funds, 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 falls more than 10 percent in five minutes, is currently being deployed 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 minutes on May 6 before sharply rebounding.

The SEC 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; editing by Andre Grenon)