State government and public health leaders have warned Americans to resist the impulse to ease social-separation measures at the first glimpse of progress, as the U.S. death toll surpassed 16,500.

Calls for heightened vigilance, countering talk from the Trump administration of reopening the economy next month, came as new evidence emerged that stay-at-home restrictions were working to flatten the curve of infections in New York state, the US epicenter of the pandemic.