RALEIGH, NORTH CAROLINA (BLOOMBERG) - US President Joe Biden warned of the risks posed by a highly transmissible and potentially deadlier coronavirus variant as he prodded Americans to get vaccinated amid a decline in the pace of inoculations.

"This new dangerous variant continues to emerge," Mr Biden said in Raleigh, North Carolina, as he warned of the delta variant, first reported in India. "It's now the most common variant in America. Unvaccinated people are incredibly vulnerable."

He added: "If you're not vaccinated, don't put it off any longer. Just do it."

Mr Biden spoke Thursday (June 24) as part of his administration's effort to re-ignite a US vaccination campaign that is slowing to a crawl.

Mr Biden's White House conceded this week that he'd miss a pair of upcoming goals - for 70 per cent of US adults to have at least one shot, and for 160 million people to be fully vaccinated, by July 4. They're on pace to hit both measures a few weeks late.

In North Carolina, 52 per cent of those 12 and older - the age group eligible to get shots - have received at least one dose and 49 per cent are fully vaccinated, according to the state.

Across the country, the pace of shots has fallen from a high of 3.4 million per day in mid-April to roughly 800,000 per day now. The rolling average of new cases has halted its decline and the number of deaths has begun to tick up again.

Mr Biden's push comes as health officials warn about the spread of the delta variant, a mutation of the coronavirus. The variant is more easily spread and possibly more harmful when you get it, Surgeon-General Vivek Murthy said on a call Thursday with reporters.

"It's a stark reminder, the delta variant, that if you are vaccinated, you are protected," Dr Murthy said. "But if you are not, the threat of variants is real and growing."

Mr Biden said his administration had successfully "built equity into the vaccination programme," saying that people of colour accounted for more than half the vaccinations given in the last month.

"The more we close the racial gap in vaccination rates, the more lives we can save," Mr Biden said.

Mr Biden has set up the July 4 Independence Day holiday as a turning point in the pandemic, holding out the prospect of a full reopening if enough people get inoculated. He plans to host 1,000 people at the White House for a celebration.

First lady Jill Biden travelled to Florida on Thursday and was in Mississippi and Tennessee on Tuesday to encourage vaccinations.

Unvaccinated adults who say they definitely won't get inoculated are more likely to be Republicans and live in rural areas, according to a Kaiser Family Foundation survey released last month.