On Monday President Joe Biden urged state politicians once again to make mask-wearing obligatory in public places.
He also promised that by mid-April 90% of American adults would be able to receive a vaccine.
The US has recorded around 60,000 new cases daily for the past week.
The director of the US public health agency, the Centres for Disease Control and Prevention (CDC), was speaking at a White House briefing when she said she was going to go "off script".
"I'm going to reflect on the reoccurring feeling I have of impending doom," Dr Rochelle Walensky said, adding "we have so much reason for hope, but right now I'm scared".
New Covid cases have reached around 60,000 a day in the past week, a rise of around 7%, according to the CDC.
Dr Walensky said she did not want the US to face another spike in cases and deaths as has happened in many European countries.
Cases have risen particularly quickly in Michigan and the country's north-east, including Connecticut and New York, according to the New York Times.
Speaking in a TV address from the White House President Biden issued a plea to state governors to re-introduce laws that require citizens to wear masks.
Coronavirus rules in the US vary state-by-state, with some governors ordering much stricter restrictions than others.
"If we let our guard down now, we can see the virus getting worse, not better," Mr Biden said.
He also spoke about the US success in its national vaccination programme and suggested it was ahead of schedule.
By 19 April, 90% of American adults will be eligible for a vaccine and will have access to a vaccination centre five miles from their homes, he promised.
Mr Biden has said all American adults will be able to register for a dose by 1 May.
More than one in five adults and nearly Americans aged over-65 are fully vaccinated, according to the CDC.
The rules on who is eligible vary by state, but in most places health care workers have long been able to get a jab, followed by the over-65s. In some states, including Georgia and Arizona, the over-16s are now able to get a jab, according to the New York Times.
Mr Biden urged Americans in the meantime to adhere to guidelines on social distancing and face coverings.
"Fight to the finish," he said. "Don't let up now."

NIGERIANS no dey fight to finish ooooo.. Some even say ONIONS and GINGER dey kill CORONAVIRUS...