Washington [US]: The US on Wednesday (local time) reported its first case of the 'Omicron' variant of the COVID-19.

The case has been detected in the US state of California, Top Medical Adviser Anthony Fauci was quoted as saying by CNN.

"The California and San Francisco Departments of Public Health and the CDC have confirmed that a recent case of COVID-19 in an individual in California was caused by the Omicron variant... The individual was a traveller who returned from South Africa on November 22 and tested positive on November 29," Fauci said.

"The infected individual, who was fully vaccinated, is experiencing mild symptoms," the medical advisor added.

Fauci also noted that people who came into contact with the individual have been notified and all have tested negative for the virus.

The new Omicron coronavirus variant has been confirmed in 23 countries and their number is expected to rise, Director-General of the World Health Organization (WHO) Tedros Adhanom Ghebreyesus said on Wednesday.

Speaking at a press briefing here, Tedros said the emergence of the Omicron variant has understandably captured global attention.

"At least 23 countries from five of six WHO regions have now reported cases of Omicron and we expect that number to grow. WHO takes this development extremely seriously," Tedros during a press conference.

"WHO takes this development extremely seriously, and so should every country. But it should not surprise us. This is what viruses do. And it's what this virus will continue to do, as we long as we allow it to continue spreading," he added.

The variant was first detected in South Africa and the WHO has called it a matter of concern for the world.

Dozens of countries have imposed travel bans on the southern African nations since the mutation was discovered at the end of last week.

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