In Florida, state police dispersed an unauthorised gathering of hundreds of people in Daytona Beach on Saturday.
In Missouri, bars at the Lake of the Ozarks were packed with revellers, who violated social-distancing rules.
US coronavirus task force chief Dr Deborah Birx said she was "very concerned" after seeing such scenes.
"We really want to be clear all the time that social distancing is absolutely critical. And if you can't social distance and you're outside, you must wear a mask," Dr Birx said on ABC's This Week on Sunday.