The new coronavirus variant Omicron is spreading across the globe at an unprecedented rate, the World Health Organization (WHO) has warned.

Cases of the heavily mutated variant have been confirmed in 77 countries.

But at a press conference, WHO chief Tedros Adhanom Ghebreyesus said it was probably in many others that had yet to detect it.

Dr Tedros said he was concerned that not enough was being done to tackle the variant.

"Surely, we have learned by now that we underestimate this virus at our peril. Even if Omicron does cause less severe disease, the sheer number of cases could once again overwhelm unprepared health systems," he said.