WHO Director-General Tedros Adhanom Ghebreyesus said that his agency, the World Bank and the International Monetary Fund backed debt relief to help developing countries cope with the pandemic's social and economic consequences.

"In the past five weeks there has been a near-exponential growth in the number of new cases and the number of deaths has more than doubled in the past week," Tedros told a virtual news conference in Geneva where the UN health organisation is based.

"In the next few days we will reach 1 million confirmed cases and 50,000 deaths worldwide," he said.

China, where the coronavirus outbreak first emerged in December, reported dwindling new infections on Wednesday and for the first time disclosed the number of asymptomatic cases, which could complicate how trends in the outbreak are read.

Its latest figures excluded 130 new sufferers of the highly contagious disease who do not show symptoms, its statistics showed.

Asked about the distinction, Dr. Maria ver Kerkhove, a WHO epidemiologist who was part of an international team who went to China in February, said WHO's definition included laboratory-confirmed cases "regardless of the development of symptoms".

"From data that we have seen from China in particular, we know that individuals who are identified, who are listed as asymptomatic, about 75 per cent of those actually go on to develop symptoms," she said, describing them as having been in a "pre-symptomatic phase".

The new coronavirus causes the respiratory disease COVID-19.

The outbreak continues to be driven by people who show signs of disease including fever and cough, but it is important for the WHO to capture that "full spectrum of illness", she added.

Tedros, referring to proposed debt relief, said: "Many countries, developing countries cannot really support their societies especially during lockdowns, especially those community members who work for their daily bread. That is why we call on the international community to have debt relief to support those countries.

"We are proposing an expedited process to support countries so their economies are not getting into crisis, (and) their communities are not getting into crisis," he said.

In the United States, people braced for what President Donald Trump warned on Tuesday could be "one of the roughest two or three weeks we've ever had in our country".

The White House projected 100,000 to 240,000 deaths in the US before the crisis is over, and Vice President Mike Pence said models for the outbreak show the country on a trajectory akin to hard-hit Italy's.

Meanwhile, European countries facing extraordinary demand for intensive-care beds were putting up makeshift hospitals, unsure whether they will find enough healthy medical staff to run them.

London is days away from unveiling a 4000-bed temporary hospital built in a huge convention centre.

In a remarkable turnabout, rich economies where virus cases have exploded are welcoming help from less wealthy ones.

Russia sent medical equipment and masks to the United States, Cuba supplied doctors to France and Turkey dispatched protective gear and disinfectant to Italy and Spain.

Worldwide, more than 900,000 people have been infected and more than 45,000 have died, according to a tally kept by Johns Hopkins University, though the real figures are believed to be much higher because of testing shortages, differences in counting the dead and large numbers of mild cases that have gone unreported.

Australian Associated Press