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.
Photo: AFP
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.
-BBC
