New Rochelle has seen "probably the largest cluster" of US cases, he said.
National Guard troops will be used to clean schools in the town and deliver food to any quarantined individuals.
The death toll in Italy, one of the countries worst hit by the virus, rose to 631 as the authorities placed the whole country in lockdown.
The latest increase in deaths in Italy - 168 - is the biggest the country has recorded in a single day, and the total number of cases went from 9,172 to 10,149.
