The World Health Organization on Wednesday warned a "toxic mix" of low vaccination coverage and low testing rates was creating fertile breeding ground for new Covid-19 variants.

The WHO said measures to stop the globally-dominant Delta variant would also hinder 'Omicron', first discovered in November in southern Africa and which has since rattled countries around the world.

WHO chief Tedros Adhanom Ghebreyesus called on countries to optimise their public health and social measures to control the spread of Covid-19.

He also said nations needed to ensure that high-risk and vulnerable individuals are fully vaccinated "immediately".

Tedros sounded a reminder that while global attention turns to 'Omicron,' the world was already grappling with the "highly transmissible, dangerous" Delta variant, which currently "accounts for almost all cases" globally.

"We need to use the tools we already have to prevent transmission and save lives from Delta. And if we do that, we will also prevent transmission and save lives from 'Omicron'," Tedros told a press conference.

"But if countries and individuals don't do what they need to do to stop transmission of Delta, they won't stop 'Omicron' either.

"Globally, we have a toxic mix of low vaccine coverage, and very low testing -- a recipe for breeding and amplifying variants.

"That's why we continue to urge countries to... ensure equitable access to vaccines, tests and therapeutics all over the world."