US President Joe Biden's administration on Sunday issued a vocal defense of a controversial Covid-19 vaccine mandate for large companies despite a court challenge that temporarily freezes the program.

"The president and the administration wouldn't have put these requirements in place if they didn't think they were appropriate and necessary," Surgeon General Vivek Murthy said on ABC talk show "This Week."

"The administration is certainly prepared to defend them."

And White House chief of staff Ron Klain, citing earlier court rulings that upheld state-level mandates, said he was "quite confident" that the mandate would ultimately be upheld.

"It's common sense," he told NBC's "Meet the Press."

The federal mandate, designed to push millions of workers at businesses with more than 100 employees into getting Covid-19 shots -- or submit to weekly testing -- is key to administration efforts to bring the coronavirus pandemic under control.

But on Saturday, the US Court of Appeals for the Fifth Circuit said several petitioners opposing the mandate had raised "grave statutory and constitutional issues." It granted a temporary stay.

Petitioners -- including five conservative US states as well as several private employers and religious groups -- have questioned the federal government's authority to issue such a nationwide order.

Murthy, in defending it, cited a long history of such requirements -- going back to George Washington requiring his troops to be inoculated against smallpox -- and said that with the country still seeing 75,000 Covid cases a day, "it's important to take every measure possible to make our workplaces safer."

And with the new mandate not set to take effect until January, Klain expressed confidence courts would by then uphold it.

The traditionally conservative Fifth Circuit court, based in New Orleans, Louisiana, ordered the government to reply to petitioners' motion for a permanent injunction by Monday evening.

Senior Biden advisor Cedric Richmond said he was "absolutely" convinced the new rule would survive judicial scrutiny.

"We're very confident we're on statutory and legal grounds," he told CBS. "This is about keeping Americans safe."

Should the suspension be upheld it would represent a major setback to one of Biden's most far-reaching efforts to secure the immunization of American workers, amid predictions of a Covid-19 winter resurgence.

Around 58 percent of the total US population was fully vaccinated as of early November, up from half in August as state, local and company mandates were announced.