The United States is working to give additional COVID-19 booster shots to Americans with compromised immune systems as quickly as possible, as cases of the novel coronavirus continue to rise, top U.S. infectious diseases expert Dr. Anthony Fauci said Thursday.

The United States is joining Germany, France and Israel in giving booster shots, ignoring a plea by the World Health Organization to hold off until more people around the world can get their first shot.

U.S. regulators need to fully authorize the COVID-19 vaccines or amend their emergency use authorizations before officials can recommend additional shots, but the U.S. Centers for Disease Control and Prevention (CDC) is working to make third doses available sooner under certain circumstances, officials said at a July meeting.

"It is extremely important for us to move to get those individuals their boosters and we are now working on that," Fauci said on a press call, adding that immunocompromised people may not be sufficiently protected by their existing COVID-19 vaccinations.

Fauci said rising cases resulting from the spread of the contagious Delta variant in the United States can be turned around with additional vaccinations.

The Biden administration has been eager to thaw opposition by some Americans, including those who distrust the government, to taking the vaccine as the highly infectious Delta variant sweeps the country.

Seven U.S. states with the lowest COVID-19 vaccination rates account for half of the country's new cases and hospitalizations in the last week, the White House said on Thursday.

The states are Florida, Texas, Missouri, Arkansas, Louisiana, Alabama and Mississippi, according to President Joe Biden's COVID-19 coordinator, Jeff Zients.

Of those, Florida and Texas account for about a third of new coronavirus cases and an even higher share of hospitalizations in the country.

COVID-19 cases are up about 43 percent over the previous week and daily deaths are up more than 39 percent, according to U.S. CDC Director Dr. Rochelle Walensky.

The United States hit a six-month high for new COVID-19 cases with over 100,000 infections reported on Wednesday.

Some 864,000 vaccinations have been given in the past 24 hours, the highest since early July, the White House said.

Zients said the Biden administration supports U.S. businesses and other institutions requiring that their employees get vaccinated.

He added that the White House is considering requiring foreign visitors to be vaccinated as it plans to eventually reopen international travel but said it had made no final decision.