As the highly contagious Delta variant of the coronavirus fuels outbreaks in the United States, the director of the Centers for Disease Control and Prevention warned on Friday that “this is becoming a pandemic of the unvaccinated.”

Cases, hospitalizations and deaths remain far below last winter’s peak, and vaccines are effective against Delta, but the C.D.C. director, Dr. Rochelle P. Walensky, urged people to get fully vaccinated to receive robust protection, pleading: “Do it for yourself, your family and for your community. And please do it to protect your young children who right now can’t get vaccinated themselves.”

The number of new virus cases is likely to increase in the coming weeks, and those cases are likely to be concentrated in areas with low vaccine coverage, officials said at a White House briefing on the pandemic.

“Our biggest concern is that we are going to continue to see preventable cases, hospitalizations and, sadly, deaths among the unvaccinated,” Dr. Walensky said. The nation surpassed 34 million cumulative cases on Friday, according to a New York Times database.

Delta now accounts for more than half of new infections across the country, and case numbers have been rising in every state. Roughly 28,000 new cases are reported each day, up from just 11,000 a day less than a month ago.

So far, data suggests that many of the vaccines — including the Pfizer-BioNTech, Moderna and Johnson & Johnson shots — provide good protection against Delta, especially against the worst outcomes, including hospitalization and death. (Receiving a single dose of a two-shot regimen provides only weak protection against the variant, however.) Nearly 60 percent of U.S. adults have been fully vaccinated, but fewer than 50 percent of all Americans have been; only those 12 and older are eligible.

“We have come a long way in our fight against this virus,” Jeffrey D. Zients, the administration’s Covid-19 response coordinator, said at the briefing.

The pace of vaccination has slowed considerably since the spring, and vaccine coverage remains highly uneven. Delta is already driving case numbers up in undervaccinated areas, including in parts of Missouri, Arkansas and Louisiana.

In mid-May, when cases were on a decline, the C.D.C. said fully vaccinated people could go maskless in most scenarios, and on July 4, President Biden hosted an event for essential workers and others at the White House to tout progress against the virus. As cases increase, Americans may have to navigate seemingly diverging messaging, with local health officials advising something potentially different than the C.D.C.’s broad guidance.

The World Health Organization recently repeated its recommendation that even vaccinated people should continue to wear masks, in part because of the global spread of Delta.

The C.D.C. has stood by its mask guidance, however, with Dr. Walensky noting the W.H.O.’s global purview and the fact that wealthy nations have snapped up so many of the available shots. She has added that local officials in the United States can opt for more stringent measures to protect the unvaccinated.

On Thursday, Los Angeles County said it was reinstating an indoor mask mandate for everyone beginning this weekend, regardless of vaccination status. On Friday, Dr. Walensky pointed out the heterogenous nature of the country and said “these decisions have to be made at the local level.”

“If you have areas of low vaccination and high case rates, then I would say local policymakers might consider whether masking at that point would be something that would be helpful for their community,” she added.

In New York City, Mayor Bill de Blasio said Friday there were currently no plans to reintroduce an indoor mask mandate for everyone citywide, nor did he think the move was needed. The city has reported a recent streak of more than 400 cases per day, up from about 200 per day on average just a few weeks ago. “We need to watch it like a hawk,” he said on a radio show, referring to the Delta variant.

Health officials are focused on hospitalizations, he said, which have remained low in recent weeks. About 53 percent of city residents are fully vaccinated, according to city data. Should hospitalization rates rise, he said, the city will adapt.

“We don’t have a plan to change course at this point,” he said. “If we see something that we need to change, we will say it immediately and will call people to arms.”

After narrowly missing a self-imposed goal of having 70 percent of adults at least partially vaccinated by July 4, the Biden administration is making a renewed push to try to reach those who have still not gotten their shots. Officials have also recently announced the creation of “surge response teams” to help hard-hit states manage Delta-driven outbreaks. Missouri and Nevada have already requested assistance.