The head of the World Health Organization in Africa on Sunday urged countries to follow the science rather than imposing flight bans in a bid to contain the new Omicron coronavirus variant.

"With the Omicron variant now detected in several regions of the world, putting in place travel bans that target Africa attacks global solidarity," said WHO regional director general Matshidiso Moeti.

"Travel restrictions may play a role in slightly reducing the spread of COVID-19 but place a heavy burden on lives and livelihoods," the WHO said in a statement.

"If restrictions are implemented, they should not be unnecessarily invasive or intrusive, and should be scientifically based," the UN body said.

The Omicron strain has cast doubt on global efforts to battle the coronavirus pandemic because of fears that it is highly infectious, forcing countries to reimpose measures many had hoped were a thing of the past.

As scientists race to determine the level of threat posed by the new strain -- particularly whether it can evade existing vaccines -- a South African doctor said dozens of her patients suspected of Omicron infection had shown only mild symptoms such as fatigue.

A long list of countries have already imposed travel restrictions on southern Africa, including key travel hub Qatar, the United States, Britain, Saudi Arabia, Kuwait and the Netherlands.