White House coronavirus task force coordinator Deborah Birx said on ABC's "This Week" Sunday that social distancing is "absolutely critical" and that if Americans can't maintain at least 6 feet from other people while gathering outside, they "must wear a mask."Why it matters: As states have begun to reopen, many Americans have flocked to beaches and outdoor restaurants in large numbers to celebrate Memorial Day weekend. Birx warned that people could be spreading the virus unknowingly, and she asked those with comorbidities to remain sheltered in place.What she's saying: "We've made it clear that there's asymptomatic spread, and that means that people are spreading the virus unknowingly," Birx said. "And this is unusual in the case of respiratory diseases in many cases.""You don't know who's infected. So we really want to be clear all the time that social distancing is absolutely critical."Asked about President Trump's call to open places of worship last week, Birx told "Fox News Sunday" that it may not be safe for people with pre-existing conditions to attend religious services and asked them to stay home.She added that there is scientific evidence that wearing a mask in indoors, including in stores, reduces the spread of respiratory droplets that may contain the virus.Go deeper ... Study: Government-enforced coronavirus social distancing works