The U.S. Securities and Exchange Commission will take up to another 60 days to decide whether to allow the sale of the Chicago Stock Exchange to a group of investors led by China-based Chongqing Casin Enterprise Group.

The SEC, which reviews proposed mergers involving exchanges to ensure they comply with federal regulations and appropriately self-police their brokerage members, said it needed more time to make the decision in a regulatory filing dated June 6.

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

The proposed deal 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.

A long-term objective of Casin Group, a privately held company that invests in real estate development and financial holdings, 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 (CFIUS), 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.

Five members of U.S. Congress, led by Representative Robert Pittenger, a Republican on the Financial Services Committee and the Congressional-Executive Commission on China, have urged the SEC to block the deal.

The lawmakers alleged that China's markets lack transparency and accused the Chinese government of being "the No. 1 state-sponsor of cyber-espionage."