Dr Anthony Fauci, the U.S. government's top infectious disease expert, warned smaller cities were likely about to see cases "take off" the way they had in New York City.
The virus has claimed almost 38,000 lives worldwide in a health crisis that is rapidly reorganising political power, hammering the global economy and the daily existence of some 3.6 billion people.
Italy and Spain saw their death tolls climb by more than 800 each on Monday, but the WHO's emergency chief said cases there were "potentially stabilizing".
On the same day, the country, however, registered the highest daily increase in the number of COVID-19 patient recoveries, which grew by 1,590 cases in 24 hours to 14,620. With a population of only 47 million people to China's 1.4 billion, Spain's tally of infections reached 85,195, an 8% rise from the previous day but down from earlier increases that had rocketed up to 20%. Secretary of Defense Mark Esper said the death of the New Jersey Army National Guardsman strengthened their resolve to work more closely with partners to stop the spread of COVID-19.
Meanwhile, the country reported 812 deaths during the period, fewer than the 838 deaths on Sunday and 832 on Saturday.
Dr. Chiara Lepora, who heads Doctors Without Borders' efforts in the virus hotspot of Lodi in northern Italy, said the pandemic had revealed critical health care issues in developed countries.
In the United States, New York's governor issued an urgent appeal for medical volunteers from across the country amid a "staggering" number of deaths from the virus, saying: "Please come help us in New York, now".
Also on Monday, Italian health minister Roberto Speranza said the nationwide lockdown in place since March 10 would be extended until "at least" April 12, nine days beyond its original deadline. As of this writing, more than 5,600 people now need intensive care treatment. The US has reported over 140,000 infections and more than 2,500 deaths so far, with New York City the worst hot spot.
Crews of workers and soldiers have been frantically building more field hospitals in the capital and surrounding towns. Nearly 15% of all those infected in Spain, almost 13,000 people, are health care workers.
The U.S. death toll from the coronavirus climbed past 3,500 Tuesday, eclipsing China's official count, as the deepening crisis in NY hit close to home for the governor, who announced with tears in his eyes that his brother, CNN anchor Chris Cuomo, has become infected.