Nasdaq, the second-largest US stock exchange after the NYSE, is the listing home of many energy companies.
All traffic through Nasdaq stopped at 12:14 p.m. in New York, the exchange said on its website.
“I would not want to speculate other than to say this is huge, everything is halted in the market,” Sal Arnuk, co-manager of trading at Themis Trading in Chatham, New Jersey, told Reuters.
Options trading was also halted, the exchange said.
Nasdaq was not immediately available for comment, Reuters said.
Nasdaq-listed stocks represent about 28% of all shares traded so far this month, according to the Wall Street Journal, citing BATS Global Markets.
The halt will likely affect the calculations of the widely tracked Dow Jones Industrial Average and the S&P 500-stock index.
Securities & Exchange Commission spokesman John Nester said the SEC is “monitoring the situation and (is) in close contact with the exchanges”.