Amid rising cases, the C.D.C. director warns the U.S. is at “another pivotal point” in the pandemic.

The director of the Centers for Disease Control and Prevention struck a new tone of urgency on Thursday about the coronavirus pandemic, warning that the United States is “not out of the woods yet” and is once again at a “another pivotal point in this pandemic” as the highly infectious Delta variant rips through communities with low rates of vaccination.

The warning from the director, Dr. Rochelle Walensky, during a briefing by members of the White House Covid-19 response team, was a marked shift from just weeks ago, when President Biden threw a big Fourth of July party on the South Lawn of the White House to declare independence from the virus.

It reflects a growing concern among administration officials that the gains they appeared to have made are being erased — and that the current surge in cases will overwhelm health systems in parts of the country where vaccination rates are low and hospitalizations are high. Still, new cases, hospitalizations and deaths remain at a fraction from their previous devastating peaks. Vaccines remain effective against the worst outcomes of Covid-19 including from the Delta variant.

Dr. Walensky pleaded with unvaccinated people to “please take the Delta variant seriously,” adding, “This virus has no incentive to let up, and it remains in search of the next vulnerable person to infect. Please consider getting vaccinated and take precautions until you do.”

White House officials also announced Thursday that they will spend $1.6 billion authorized by Congress as part of the American Rescue Plan on testing and other mitigation measures in “high risk congregant settings,” including homeless shelters and prisons. And they said they would send $100 million to rural health clinics to support vaccine education and outreach efforts in those communities, where vaccination rates are generally low.