Scotland's first minister has said it is not time to "press the panic button" over a new Covid variant circulating in southern Africa.
Nicola Sturgeon said the B.1.1.529 strain was the most serious development for some time but much was yet unknown.
Six African countries have been added to the UK's red travel list, and new quarantine rules have been announced.
Ms Sturgeon said there was no immediate need to revisit the recent decision not to extend the vaccine passport scheme.
The World Health Organization (WHO) has now designated the new form of the virus a "variant of concern" and given it the name Omicron.
Most cases have been detected in South Africa but it has also been identified in Hong Kong, Botswana, Namibia, Israel and most recently in Belgium.
The Omicron variant is described as "highly mutated" with numerous changes to the spiked protein structure, and the WHO has said there is preliminary evidence it carries a greater risk of reinfection.
Ms Sturgeon told the BBC: "I don't think it's time to press the panic button, but it's probably the most serious development and the most worrying development we've had in the Covid journey for some months.
"Certainly the early data from the southern part of Africa is of significant concern, but we need to see more data and more analysis. So we are monitoring it very, very carefully and closely."
She said her government's decision this week not to extend Scotland's vaccine passport was unchanged, but would be kept under review.
In the meantime she said Scotland and other parts of the UK had taken "highly precautionary" actions on isolation for people arriving in Scotland from certain countries.
"We shouldn't panic at this stage. But if we've learned anything over the past two years it is that we need to take all of these developments very seriously.
"It would be a real blow to everybody if we suddenly found ourselves dealing in Scotland and the UK with a powerful new variant, and we need to guard against that."
The first minister was speaking to BBC Scotland political editor Glenn Campbell as the SNP opened its annual conference.
The four-day conference, which is being held entirely online, will conclude with Ms Sturgeon's address to delegates on Monday.
From 04:00 on Saturday anyone coming to Scotland who has been in one of the red list countries in the 10 days prior to arrival, must go into managed quarantine at a hotel.
Anyone arriving before then should isolate at home, or where they are staying, and all arrivals from these countries must take two PCR tests during their quarantine period.
Travellers who have already arrived in the past 10 days will be asked to take two tests but will not have to self-isolate unless they test positive.
Health Secretary Humza Yousaf said there was a "significant probability" that more countries would be added to the red list.
Public health expert Prof Linda Bauld said there nothing to suggest the variant had arrived in the UK yet - but that it was entirely possible that it had.
"Unfortunately, these things are only really identified when they are already spreading - and you can see the number of African countries that are affected," she said.
She said it could take a "couple of weeks" to get a better indication of what threat it might pose.
Devi Sridhar, professor in global public health at the University of Edinburgh, said the new travel restrictions would "buy time" to roll out booster vaccines while research is carried out into the new variant.
She told the BBC's Good Morning Scotland programme the priority for scientists would be to establish if the new variant was more transmissible than the delta variant - and therefore likely to become the dominant strain - and to what extent existing vaccinations remained effective.
"Of course, I think the worry is it could actually be here - but we know sequencing is very good in the UK so hopefully if there are confirmed cases, they will pick them up quite quickly," she said.
"But I don't think we can stop importation fully - you can't stop it but you can delay it and buy time - and that delay gives you time to get more boosters out to people and figure out what's your game plan."