The World Health Organization (WHO) is due to hold a special meeting on Friday to consider the significance of a rapidly spreading new coronavirus variant in South Africa.
Experts in Geneva will decide whether it should be designated a variant of concern.
A senior WHO official said it could take weeks to establish whether the high number of mutations had implications for the effectiveness of existing vaccines.
It comes as Britain imposes a temporary ban on all flights from South Africa and five neighbouring countries later on Friday.
No cases have been discovered in the UK.
South African experts say the variant has spread rapidly among students in Pretoria.
