A US federal appeals court Saturday halted a vaccine mandate by President Joe Biden's administration that is intended to push millions of workers at businesses with more than 100 employees into getting Covid-19 shots.

The New Orleans-based US Court of Appeals for the Fifth Circuit said that because petitioners "give cause to believe there are grave statutory and constitutional issues with the Mandate, the Mandate is hereby STAYED pending further action by this court."

The traditionally conservative court ordered the government to reply to the motion for a permanent injunction by 5:00 pm Monday.

Petitioners include five strongly Republican US states -- Texas, Louisiana, South Carolina, Utah and Mississippi -- as well as several private companies and religious groups.

If upheld, the suspension would represent a stunning setback to one of Biden's most far-reaching and high-impact efforts to secure the widespread immunization of American workers ahead of a winter when a resurgence of Covid-19 is expected.

 'Abuse of authority'

There was no immediate reaction from the Biden administration.

But Texas Governor Greg Abbott, a conservative Republican who has opposed mask and vaccine mandates, applauded the decision in a post on Twitter.

"Emergency hearings will take place soon," he said. "We will have our day in court to strike down Biden's unconstitutional abuse of authority."

Vaccine mandates have a long history in the US, but have generally been issued by cities or states.

At issue here is the constitutionality of a US president issuing such a sweeping nationwide mandate.

Texas attorney general Ken Paxton, among those filing suit to block the Biden mandate, said Saturday on CNN that the president had exceeded his authority.

"The fight is not over," Paxton tweeted earlier, "and I will never stop resisting this Admin's unconstitutional overreach!"

 Where mandates worked

Around 58 percent of the total US population was fully vaccinated as of early November, up from half in August as mandates began to be announced.

Biden on Thursday had set a January 4 deadline for employees in large companies to be fully vaccinated -- a rule the administration said would affect more than two-thirds of the country's workforce.

"Vaccination is the single best pathway out of this pandemic," he said.

"Businesses have more power than ever before to accelerate our path out of this pandemic, save lives, and protect our economic recovery."

Mandates announced earlier by several major employers have had impressive results.

They included United Airlines, which told its 67,000 US-based staff to get the jab or face termination.

By Thursday, 2,000 had sought medical or religious exemptions and all but a handful of the rest had complied.

Tyson Foods, one of the biggest meatpacking companies in the world, announced in August that all of its 120,000 staff would need to be vaccinated.

Around 96 percent have fallen into line, the New York Times reported.

The US military, which has told all serving personnel they must be vaccinated -- or face consequences -- reports rates above 95 percent in all services.