The head of the World Health Organization (WHO) voiced concern on Tuesday that some states are introducing blanket measures aimed at the Omicron coronavirus variant that he said were "not evidence-based or effective on their own" and were penalizing southern African countries that reported the strain so quickly.

Tedros Adhanom Ghebreyesus, in a speech to the WHO's 194 member states, urged them to take "rational, proportional risk-reduction measures" in keeping with the agency's 2005 International Health Regulations.

"We still have more questions than answers about the effect of Omicron on transmission, severity of disease, and the effectiveness of tests, therapeutics and vaccines," Tedros said in remarks posted on the WHO website.