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The New York Stock Exchange (NYSE) has said share trading was suspended due to a technical fault, not a hack.

It said it would give more details as soon as possible. All open orders will be cancelled, Reuters said.

The Nasdaq index reported no technical issues and said it continues to trade NYSE-listed stocks.

The Wall Street Journal website and United Airlines also reported technical problems, although US authorities said the issues appear unrelated.

"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, and the Securities and Exchange Commission said it was "closely monitoring the situation".

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."

'Not nefarious'

Meanwhile, the WSJ.com site and United Airlines reported technical difficulties on Wednesday.

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.

US Homeland Security Secretary Jeh Johnson said the issues reported by United and the NYSE were apparently not related to "nefarious" activity.

"I have spoken to the CEO of United, Jeff Smisek, myself. It appears from what we know at this stage that the malfunctions at United and the stock exchange were not the result of any nefarious actor," Mr Johnson told reporters in Washington.

"We know less about the Wall Street Journal at this point, except that their system is in fact up again," he added.