The New York Stock Exchange has resumed trading after shutting for more than three and a half hours because of what it calls an internal technical issue.
The NYSE said it would provide more details as soon as possible.
The Nasdaq index reported no technical issues and said it continues to trade NYSE-listed stocks.
The Wall Street Journal website also experienced technical problems.
"We're currently experiencing a technical issue that we're working to resolve as quickly as possible," a NYSE spokeswoman said in a statement.
"We will be providing further updates as soon as we can, and are doing our utmost to produce a swift resolution, communicate thoroughly and transparently, and ensure a timely and orderly market re-open."
However, in a tweet later the NYSE said the problem was an internal technical issue, not a cyber attack. The White House said that President Barack Obama had been briefed.
US markets were down before the halt in trading, which started just after 11:30am ET, as a fall in Chinese markets added to concerns about its impact on global economic growth.
One trader on the NYSE floor, Mark Otto, of J Streicher & Co, told Reuters: "It's under control. We're just waiting for word. There's no sign of panic at all.
"We're waiting to hear word on if there's going to be a reopening, and when it is or any more details."
Meanwhile, the WSJ.com site and United Airlines reported technical difficulties.
WSJ.com went down completely earlier in the day, but there was a partial recovery at about midday. A message on the website's homepage read: "WSJ.com is having technical difficulties. The full site will return shortly."
United Airlines has also reported that a computer error has forced it to ground its flights in the US for the second time in recent weeks.
The carrier blamed a "network connectivity issue" for the latest fault.