US stocks continued to fall on Thursday morning after President Donald Trump's Wednesday night Oval Office speech aimed at calming markets.

On opening, all three major Wall Street indices, The S&P 500, Dow Jones Industrial Average, and Nasdaq all plunged, triggering the automatic 'circuit breakers' that halt trading.

Trading stopped for 15 minutes to establish order in the market, and resumed at 9.50am.

Markets continued to fall with the Dow down almost 9 per cent by 10am.

'Circuit breakers' kick in when the market drops by specified percentages against the prior day's S&P 500 closing price. There are three levels.

A 7 per cent decrease halts trading for 15 minutes. Another 15 minute pause happens if the market falls 13 per cent. If the market drops by 20 per cent, then trading is halted for the rest of the day.

The current system was put in place in 2013 after the previous system failed to prevent the flash crash of May 2010.

The S&P 500 and Dow Jones are now in bear territory (down 20 per cent from their peak), ending an 11-year bull market run — the terms used to describe the general trends in the stock market — a bull market is on the rise, a bear market trends downwards.

President Trump's Oval Office address on Wednesday night did little to calm fears about the medical, social and economic impacts of the coronavirus.

Rather than addressing the desperate need for wider testing, he banned travel from Europe to the US for non-US citizens and permanent residents, and touted loans from the Small Business Administration and deferments in tax payments to help the economy.