US news network CNN has sacked three employees for going into an office without having been vaccinated against Covid, US media say.

It is one of the first examples of a US firm firing staff for breaching a company vaccination mandate.

It is legal in the US for firms to require employees to be vaccinated.

Many large firms - including Facebook and Google - say they will require employees to be vaccinated when offices fully re-open in the months ahead.

CNN chief Jeff Zucker mentioned the dismissal in a company memo sent on Thursday and seen by several US media outlets.

Vaccination is mandatory for anyone reporting in the field, working with any other employees or going into an office, he is quoted as saying in the memo.

"Let me be clear - we have a zero-tolerance policy on this," Mr Zucker, chairman of news and sports for WarnerMedia, is quoted as saying.

In May the US government said it was legal for employers to require staff attending the workplace in person to be vaccinated against Covid-19.

Major airlines Delta and United Airlines are requiring new employees to show proof of vaccination, while investment bank Goldman Sachs is requiring its employees to disclose their vaccination status, although does not require staff to be vaccinated, AP reports.

President Biden has ordered two million federal employees to show proof of vaccination or be subject to mandatory testing and mask-wearing.