FILE PHOTO - The U.S. Securities and Exchange Commission logo adorns an office door at the SEC headquarters in Washington, June 24, 2011. REUTERS/Jonathan Ernst

NEW YORK (Reuters) - Eleven members of Congress asked the U.S. Securities and Exchange Commission on Monday to stop the sale of the Chicago Stock Exchange to a group led by China-based investors, saying the regulator lacks the ability to monitor the foreign buyers.

The proposal to sell privately owned CHX for an undisclosed amount to a consortium led by Chongqing Casin Enterprise Group (CCEG) has drawn attention because it would be the first time a U.S. exchange has been bought by Chinese investors. There are also U.S. investors in the group.

“With little or no insight and transparency into government-dominated Chinese markets, the SEC will be unable to monitor the ownership structure of CCEG after approval, leaving CHX open to undue, improper, and possibly state-driven influence,” the Republican and Democratic lawmakers said in a letter to the SEC.

CHX declined to comment.

The SEC, which reviews proposed mergers involving exchanges to ensure they comply with federal regulations and appropriately self-police their brokerage members, said in early June it would take up to another 60 days to make a decision on the sale.

CHX is a niche player in the U.S. equities market, executing less than 0.5 percent of U.S. stock transactions.

Casin Group, a privately held company that invests in real estate development and financial holdings, said its long-term goal is to list Chinese companies in the United States through CHX, which has locations in Chicago and New Jersey.

The Committee on Foreign Investment in the United States, which scrutinizes deals for potential national security concerns, approved the planned sale in December, but the SEC still needs to sign off for it to go ahead.

CHX is one of 13 U.S. stock exchanges, including Intercontinental Exchange Inc's ICE.N New York Stock Exchange, Nasdaq Inc NDAQ.O, and CBOE Holdings CBOE.O-owned Bats.