The current coronavirus outbreak is the biggest challenge for the world since World War Two, United Nations Secretary General António Guterres has warned.
He said it could bring a recession "that probably has no parallel in the recent past".
Mr Guterres was speaking at the launch of a UN report on the potential socioeconomic impact of the outbreak.
The number of confirmed cases around the world is now nearing 860,000, with more than 42,000 deaths.
Deaths from the disease in the US have now passed 3,800 - higher than that the 3,300 recorded in China, where the outbreak began late last year.
More than 189,000 people in America have been infected, according to the data compiled by Johns Hopkins university.
About three out of four Americans are now, or about to be, under some form of lockdown, as more US states tighten measures to fight the coronavirus that causes the Covid-19 disease.
In the last 24 hours in Spain, one of the worst hit countries, there has been 849 deaths - the highest number of fatalities in a single day.
In the UK, a total of 1,789 people died on 30 March - a rise of 381, officials say. King's College Hospital Trust in London said among the victims was a 13-year-old boy.
What did Mr Guterres say?
Speaking at the UN headquarters in New York City, Mr Guterres said: "The new coronavirus disease is attacking societies at their core, claiming lives and people's livelihoods.
"Covid-19 is the greatest test that we have faced together since the formation of the United Nations."
The UN chief called for "an immediate co-ordinated health response to suppress transmission and end the pandemic".
Mr Guterres urged the industrialised nations to help those less developed, or potentially "face the nightmare of the disease spreading like wildfire".
The UN report estimates that up to 25 million jobs could be lost around the world as the result of the outbreak.
It also projects an up to 40 per cent "downward pressure" on global foreign direct investment flows.
Grim death toll projections in US
US President Donald Trump had already announced the need to significantly extend social distancing guidelines beyond the two weeks just completed, saying people would need to stay at home as much as they possibly could for another month.
At Tuesday's White House briefing he turned the microphone over to his medical experts to help persuade Americans to comply.
They explained the aim was to bring down the number of new cases in the hotspots, especially New York, and prevent runaway outbreaks in other states.
Infectious disease expert Dr Anthony Fauci said steps taken so far had already begun to have an effect, and now was not the time to take the foot off the accelerator.
But even with strict measures, the White House is projecting between 100,000 and 240,000 Americans will die of the coronavirus pandemic.
New York will be the first to feel the brunt of that. It's focused on preparing enough beds and getting enough medical equipment for when the number of cases reaches a peak - in about two weeks' time.
What has been happening around the world?
Italy remains the worst affected country in the world, and it's figures continue to describe a frightening situation.
Tuesday's numbers show the official death toll rose by 837 deaths to 12,428, even more than the 812 deaths recorded on Monday. And 2,107 new cases were recorded, which is greater than the 1,648 people found to be infected the day before (but less than equivalent numbers last week).
France has recorded 499 people have died in hospitals in the last 24 hours, bringing the country's total to 3,523. This is the highest day-on-day rise in such deaths in France since the outbreak began.
A 12-year-old girl has died in Belgium. She is thought to be the youngest Covid-19 victim in Europe. Belgium has recorded 705 people have died from Covid-19 during the outbreak.
In Russia, lawmakers have passed an "anti-virus" package of laws, including up to seven years in prison for serious violations of quarantine rules.
In India, authorities are searching for hundreds of people who attended a religious event in the capital that has set off several Covid-19 clusters.
In the Middle East, there are warnings that the outbreak is a ticking time-bomb for the unstable region.
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