Customers look at sanitising wipes on the shelves of a pharmacy in New York. File photo/AP

The number of novel coronavirus cases in the US surged past 1,000 on Tuesday, after public health experts criticised authorities for downplaying the epidemic and lagging behind in testing efforts.

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At least 28 people have died and 1,025 people have been infected, according to a running tally by Johns Hopkins University — nearly double the 550 total confirmed cases the day before.

The rise is linked to an expansion in testing as the bulk of diagnoses have shifted from federal to state laboratories.

Supporters of Bernie Sanders leave after the rally was cancelled over COVID-19 concerns in Cleveland, Ohio. AFP

Epidemiologists have said faulty test kits coupled with a diagnostic strategy that initially targeted too few people allowed the disease to spread beyond US authorities' ability to detect it.

The failings had contributed to the virus taking root across the country, academics from Johns Hopkins University and Stanford University wrote in the Journal of the American Medical Association (JAMA) on Monday.

Vice President Mike Pence defended the government's response at a White House briefing and said that "a million tests are in the field." He said that more would be added as the government partnered with private companies.

Just over 8,500 tests had been performed as of Monday, according to the latest figures from the Centers for Disease Control and Prevention (CDC).

This handout illustration image shows SARS-CoV-2 (yellow) — also known as 2019-nCoV, the virus that causes COVID-19 — isolated from a patient in the US, emerging from the surface of cells (blue/pink) cultured in the lab. AFP

South Korea announced its first case on the same day as the United States; it tested more than 189,000 people in the same period, Business Insider reported.

The JAMA report authors wrote that the only test initially authorised was one developed by the CDC.

It relied on the same technology as one authorised by the World Health Organisation (WHO) and deployed around the world -- except that a fault meant the CDC kit was returning inconclusive results.

It was not until February 29, the date of the first US death and more than a month after the first confirmed US case, that the Food and Drug Administration lifted a ban on state laboratories developing their own kits based on WHO's tests.

Agence France-Presse