The Delta variant of COVID-19 is now the dominant strain worldwide, accompanied by a surge of deaths around the United States almost entirely among unvaccinated people, U.S. officials said on Friday.

U.S. cases of COVID-19 are up 70% over the previous week and deaths are up 26%, with outbreaks occurring in parts of the country with low vaccination rates, U.S. Centers for Disease Control and Prevention Director Rochelle Walensky said during a press briefing.

The seven-day-average number of daily cases is now more than 26,000, more than twice its June low of around 11,000 cases, according to CDC data.

The starkest uptick in cases has occurred in Arkansas, Florida, Louisiana, Missouri and Nevada, said White House COVID-19 response coordinator Jeff Zients. All those states have below-average vaccination rates.

"This is becoming a pandemic of the unvaccinated," Walensky said, adding that 97% of people entering hospitals in the United States with COVID-19 are unvaccinated.

She said an increasing number of counties around the United States now exhibit a high risk of COVID-19 transmission, reversing significant declines in transmission risk in recent months.

Around one in five new cases have occurred in Florida, Zients said.

The Delta variant, which is significantly more contagious than the original variant of COVID-19, has been detected around 100 countries globally and is now the dominant variant worldwide, top U.S. infectious disease expert Anthony Fauci said.

"We are dealing with a formidable variant" of COVID-19, Fauci said during the call.

Walensky urged unvaccinated Americans to get COVID-19 shots, and said Pfizer Inc's and Moderna Inc's vaccines have proven to be especially effective against the Delta variant.

She said people should get the second dose of vaccine even if they have passed the recommended window of time for receiving it.

Around 5 million people have been vaccinated in the United States in the past 10 days, Zients said, including many in states that so far have had lower vaccination rates.

"In the past week, the five states with the highest case rates - Arkansas, Florida, Louisiana, Missouri and Nevada - had a higher rate of people getting newly vaccinated," he said.

Delta outbreaks may be boosting demand for vaccinations in harder-hit states, Michael Newshel, a healthcare analyst at Evercore ISI, said in a note to clients. He added that residents of Utah and California are also seeking shots at an accelerated pace.

Zients added that the United States has enough vaccines on hand to give booster vaccines but is still working to determine if boosters are needed.