Birx, who heads the White House coronavirus task force, told CNN's "State of the Union" that local virus mitigation procedures were starting to work, but added that "what we are seeing today is different from March and April".
"It is extraordinarily widespread. It's into the rural as equal urban areas," she added. "To everybody who lives in a rural area, you are not immune or protected from this virus."
"We are in a new phase," Birx said.
She added that sticking to health and safety guidelines -- such as wearing a mask, maintaining personal hygiene and observing social distancing -- was crucial.
As of Sunday, the US had tallied more than 4,665,000 cases of COVID-19 and 154,834 deaths, according to Johns Hopkins University, making it by far the hardest-hit country in the world.
The US notched 47,508 new cases on Sunday -- a slight dip from the previous five days, which had all recorded more than 60,000 new infections.