US stocks have plummeted as fears about the effect of the coronavirus outbreak on economic growth flared even as US officials pledged they were stepping up efforts to safeguard Americans from the virus' spread.
A day after President Donald Trump told Americans that the risk from coronavirus remained "very low", the S&P 500 logged its fastest drop of at least 10 per cent in history in a sixth straight day of declines.
The broad market average has lost 12 per cent since hitting a record close on February 19.
Two US officials told Reuters the Trump administration was considering invoking special powers to rapidly expand domestic production of protective masks and clothing to help combat the coronavirus in the United States.
During an interagency call on Wednesday, officials from Health and Human Services (HHS) and the Department of Homeland Security discussed the possibility of invoking the Defense Production Act for the manufacture of "personal protective equipment" that can be worn by healthcare workers to prevent infection, according to a DHS official.
Amid a flurry of sometimes contradictory messages from Washington, HHS Secretary Alex Azar late on Thursday proclaimed that "everyday Americans don't need to be worried" about coronavirus. He was quick to add, "But that can change."
During a White House ceremony celebrating Black History Month, Trump countered congressional Democrats' charges that administration preparations were lagging.
"Fortunately we made the early moves," Trump said. "It's working out very professionally. We're doing a tremendous job."
Democrats hammered administration officials at two congressional hearings, demanding more details on the public health response.
Those demands took on urgency after officials confirmed a woman in northern California with coronavirus was not tested for more than week after she was admitted to the University of California, Davis Medical Center.
Robert Redfield, director of the Centers for Disease Control and Prevention, told lawmakers the agency revised its criteria for who should be tested for the coronavirus and was shipping more test kits to states.
California had only about 200 test kits, an "inadequate" number but had been in "constant contact with federal agencies" who have promised to send a fresh supply in coming days, Governor Gavin Newsom told a news briefing earlier.
Azar told a House of Representatives committee on Thursday at least 40 public health labs should now be able to test specimens for coronavirus and that could more than double as soon as Friday.
The number of confirmed US cases is still relatively small at 60, most of them repatriated American passengers from the Diamond Princess cruise ship docked in Japan.
Trump, a Republican, is seeking $US2.5 billion ($A3.8 billion) from Congress to boost the government's response. Senate Democratic Leader Chuck Schumer called for $US8.5 billion.
Trump has been increasingly alarmed by market reaction to the outbreak, which he considers a barometer of the economy's health and sees as important to his re-election in November.