The World Health Organization called Tuesday for countries to keep calm and take "rational" measures in response to the new, fast-spreading Covid variant Omicron, which has sparked global panic.

"We call on all member states to take rational, proportional risk-reduction measures," WHO chief Tedros Adhanom Ghebreyesus said in a briefing to countries.

"The global response must be calm, coordinated and coherent."

First reported to the WHO in southern Africa less than a week ago, the new strain has rapidly spread across continents, with dozens of countries announcing travel restrictions.

The UN health agency has cautioned against such restrictions, fearing that blocking travel from countries where new variants are first spotted could be unfair and dissuade surveillance.

"I thank Botswana and South Africa for detecting, sequencing and reporting this variant so rapidly," Tedros said, adding that it was "deeply concerning to me that those countries are now being penalised by others for doing the right thing."

Scientists in South Africa said they had detected the new variant with at least 10 mutations, and WHO has cautioned that it poses a "very high" risk globally.

'More questions than answers'

At the same time, Tedros stressed that it remains unclear how dangerous the variant is.

"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," he said.

The WHO chief said it was understandable that countries wanted to protect their citizens "against a variant that we don't yet fully understand".

"But I am equally concerned that several member states are introducing blunt, blanket measures that are not evidence-based or effective on their own, and which will only worsen inequities."

Tedros also highlighted the glaring vaccine inequity that has seen abundant supplies in wealthy countries, which are rolling out booster shots, as even many of the most vulnerable in poorer nations are still waiting for their first jabs.

Health experts have long warned that allowing Covid to spread unabated in some places dramatically increases the chances that new, more dangerous variants could emerge, placing the entire world at risk.

"The longer we allow the pandemic to drag on, by failing to address vaccine inequity, or to implement public health and social measures in a tailored and consistent way, the more opportunity we give this virus to mutate in ways we cannot predict or prevent," Tedros said.\