The US Centers for Disease Control and Prevention says new cases of coronavirus infections are increasing across the country, especially among young adults.
CDC Director Rochelle Walensky raised the concerns about the surge at a news conference on Monday.
The CDC says the number of daily new cases rose to about 64,000 on average in the past week, compared with below 60,000 in early March.
Walensky said the increases are due in part to more highly transmissible variants and the fact that the new cases are predominantly among younger adults.
She said, "We are learning that many outbreaks in young people are related to youth sports and extracurricular activities."
Walensky called on people to stick to measures to prevent the spread of the virus, such as wearing face masks and social distancing especially as in-person classes have resumed at many US schools.
Many US states have eased restrictions on restaurants and reopened economic activity as the government speeds up the vaccine rollout. But only 18 percent of the population have been fully vaccinated.
Experts are expressing concerns that the current uptrend of cases may lead to the "fourth wave" of infections.