WASHINGTON (BLOOMBERG) - The Biden administration is sending response teams to unvaccinated pockets of the US in an attempt to combat the spread of the highly transmissible Delta Covid-19 variant.

US health officials will boost testing, provide therapeutics and deploy federal personnel where needed and requested, said Jeffrey Zients, President Joe Biden's Covid-19 response coordinator.

Staff will come from the Health and Human Services Department, the Centres for Disease Control and Prevention and the Federal Emergency Management Agency.

One thousand US counties have Covid-19 vaccination coverage of less than 30 per cent, and the CDC is seeing increasing rates of the disease, primarily in the Southeast and Midwest, director Rochelle Walensky said on Thursday (July 1) in a White House briefing.

"As the delta variant continues to spread across the country, we expect to see increased transmission in these communities, unless we can vaccinate more people now," she said.

Biden warned June 27 that unvaccinated people are "incredibly vulnerable" to Delta.

"If you're not vaccinated, don't put it off any longer," he said in Raleigh, North Carolina. "Just do it."

Roughly two of three adult Americans in the US have received at least one dose of the vaccine, but the pace of shots is falling. While Covid-19 cases and deaths have fallen dramatically since the introduction of vaccines, the seven-day average of new cases rose 10 per cent increase over last week's level.

The Delta variant, first identified in India, makes up 26 per cent of positive Covid-19 samples sequenced and is likely to become the second most prevalent lineage across the country, CDC spokesman Jade Fulce said in an email. In some regions of the US it has become the leading variant, Fulce added.

In Thursday's briefing, Walensky said she expect the Delta variant to eclipse Alpha in US in the coming weeks.

As the Delta variant is likely to continue its spread across the US, vaccination is the main tool to reduce the impact and burden of the this variant, Moritz Kraemer, a University of Oxford epidemiologist, said in an email.

"Any effort to vaccinate pockets of yet unvaccinated populations are important. However, balancing these efforts with continued global supply of vaccinations is important."