The new policy amounts to a recognition by the Biden administration that the government - the nation's biggest employer - must do more to boost sluggish vaccination rates, as coronavirus cases and hospitalisations rebound, driven largely by the spread of the more infectious delta variant.

Biden has placed the blame for the resurgence of the virus squarely on the shoulders of those who aren't vaccinated.

"The pandemic we have now is a pandemic of the unvaccinated," Biden said on Wednesday during a visit to a truck plant in Pennsylvania, where he urged the unvaccinated to "please, please, please, please" get a shot.

The administration on Wednesday was still reviewing details of the expected guidance, and significant questions about its implementation and scope remained.

An anonymous source says it won't be a vaccine mandate for federal employees and those who decide not to get vaccinated aren't at risk of being fired.

The source said the move was intended to provide an example for private companies to follow.

The new guidance on vaccinations for federal employees reflects the reality that Biden's national vaccination drive has fallen short of his goals. Public opinion seems to have hardened around the vaccines, with a recent poll from The Associated Press-NORC Centre for Public Affairs Research finding that among American adults who have not yet received a vaccine, 35 per cent say they probably will not, and 45 per cent say they definitely will not.

About 60 per cent of American adults have been fully vaccinated. Biden missed his goal of having 70 per cent of adults get at least one shot by July 4. The latest figure is 69.3 per cent.

Federal workers and contractor employees are dispersed throughout the nation, including many in states where vaccine scepticism runs high. New York University public service professor Paul Light suggested the new guidance from the Biden administration could help boost vaccination rates in states where there's been significant resistance.

"If the federal government were to say that everybody who works for the government directly or indirectly must be vaccinated, that's a massive footprint," Light said.

He estimated that the federal government directly employs 2.2 million full-time civil servants, plus 1.4 million active-duty military personnel and about 500,000 workers in the US Postal Service. Private contractor employees working on federal jobs number about 5 million, and there are 1.8 million other people employed under federal grants.

The Biden administration may have to grapple with legal challenges to the latest guidelines.

The federal workplace is governed by layers of rules and regulations, so private employers as well as state and local governments will be looking at the White House vaccination policy to signal how far they can go without triggering resistance from employees or even lawsuits.

The Justice Department and the federal Equal Employment Opportunity Commission have both said no federal laws prevent businesses from requiring vaccinations as a condition of employment.

Among examples from major companies, Delta Air Lines and United Airlines are requiring new employees to show proof of vaccination. Goldman Sachs is requiring its employees to disclose their vaccination status but is not mandating they be vaccinated.

If an employer does set a hard requirement, employees can ask for an exemption for medical or religious reasons under federal civil rights laws.

Australian Associated Press