United States President Joe Biden, in a shift to a more forceful and even exasperated tone towards vaccine scepticism, has outlined a slew of incentives to coax reluctant Americans, including paying them US$100 (S$135) for getting fully vaccinated.

The US is facing a surge of Covid-19 cases driven by the Delta variant riding on a large remaining pool of unvaccinated people, which threatens to reverse the gains of the last six months.

On Thursday, the President signalled that the administration has had enough of vaccine scepticism.

Vaccinations had "hit a brick wall," he said.

"America is divided between the majority of eligible people who are vaccinated and those who are not. And I understand that many of you in the majority are frustrated with the consequences of the failure of the minority to get vaccinated," he added.

Mr Biden directed all federal employees and onsite contractors to show that they are vaccinated, or otherwise submit to regular coronavirus testing, wear masks, and forgo travelling for work.

The federal government employs more than four million Americans, including over two million in the federal civilian workforce, throughout the country and abroad.

The new directive does not amount to an order to get vaccinated or lose their jobs, but it does make it very inconvenient for them not to be vaccinated.

The President also said the Pentagon must determine how and when the vaccine will be mandated for all service members.

Mr Biden's televised address was a marked shift from just a month ago, when a sense of optimism prevailed as vaccination rates climbed and hospitalisations fell.

Since then, vaccination rates have stalled at just over half the population, causing increasing friction.

The bolder moves by the White House will inevitably pour fuel on the political and cultural divide between those - mostly in the more heavily Republican states - reluctant to take the vaccine or mask up and those who have, and do.

"Right now, too many people are dying, or watching someone they love die," Mr Biden said. "Studies showed that over 99 per cent of Covid-19 deaths had been among the unvaccinated."

"If in fact you are unvaccinated, you present a problem," he added.

As at Monday, the four-week Covid-19 case count more than doubled from the previous four weeks.

Cases and hospitalisations - even of the very young - are rising everywhere, but fastest in areas with low vaccination rates.

The steepest case increases have been in the south and south-east, where the states of Florida, Tennessee, Louisiana, Mississippi and South Carolina are dealing with big outbreaks. Vaccination rates lag in all of them.

Mr Biden encouraged small and medium-sized businesses to take advantage of federal programmes covering the cost of paid leave for staff to use to get vaccinated. This would be extended to cover paid leave for employees to take family members to get vaccinated.

"We are going to use the full set of carrots and sticks," White House Chief of Staff Ron Klain told MSNBC after the President concluded his remarks.

Mr Biden said that in the six months since he took office on Jan 20, 164 million Americans have been vaccinated. The economy is also recovering; separately, the Department of Commerce announced on its website that it grew 6.5 per cent in the second quarter of the year.

"We still have a lot of work to do" with the economy, but need "to stay ahead of this virus", Mr Biden pleaded. "Please, exercise responsible judgment. Get vaccinated."

Some conservative voices are slowly coming around to encouraging vaccination, and more organisations and cities have announced vaccine and masking requirements.

In Washington, Mayor Muriel Bowser, a Democrat, announced that from early tomorrow morning, everyone over the age of two must wear a mask indoors regardless of vaccination status.

New measures

President Joe Biden has announced new measures to encourage Americans to get vaccinated. Here is a look at some of them.

1. All civilian federal workers and onsite contractors have to attest that they have been fully vaccinated or they will be required to wear a mask on the job, physically distance themselves from other workers and visitors, comply with a weekly or twice-weekly screening test requirement and be subject to restrictions on official travel. The requirements apply to some 2.1 million federal employees, and millions more who work on federal contracts, but not to the estimated 650,000 permanent and temporary US Postal Service employees.

2. Visitors of federal buildings or those attending federally sponsored conferences must attest that they have been vaccinated or show that they have received a negative Covid-19 test within the last three days.

3. The Department of Defence will review "how and when they will add the Covid-19 vaccination to the list of required vaccinations for members of the military". The US military has 1.3 million enlisted personnel and officers, and more than 700,000 civilian members.

4. Federal workers will get paid time off to get vaccinated or to accompany children being vaccinated. They will also get time off to recover from the effects of a vaccine shot. Small and medium-sized businesses will be reimbursed for offering employees paid leave to get family members, including children, vaccinated.

5. State and local governments have been urged to offer US$100 (S$135) to people who get themselves vaccinated, a step New York City has already taken. They can use federal funds to pay for the incentives, the US Treasury said.

6. School districts nationwide should host at least one pop-up vaccination clinic over the coming weeks to get students vaccinated.