"I am highly concerned that Omicron, being more transmissible, circulating at the same time as Delta, is leading to a tsunami of cases," WHO chief Tedros Adhanom Ghebreyesus told a press conference.

"This is and will continue to put immense pressure on exhausted health workers, and health systems on the brink of collapse."

He said the pressure on health systems was not only due to new coronavirus patients, but also large numbers of health workers falling ill with Covid.

"The unvaccinated are many times more at risk of dying from either variant," he added.