Two people in the UK have been found to be infected with the new COVID-19 variant, Omicron, Health secretary Sajid Javid said on Saturday.

The World Health Organization on Friday declared Omicron a variant of concern, placing it in the same category as the Delta variant, which caused waves of infections to sweep worldwide.

According to Javid, the two individuals are in self-isolation and the cases are linked. The respective households are also in self-isolation while additional tests are being carried out.

He added there will be targeted testing in the areas where the cases were found. The areas in question are Chelmsford and Nottingham.

The connection to the cases has been traced to southern Africa.

UK prime minister Boris Johnson is expected to take part in a news conference later on Saturday with senior health officials "to set out further measures", local media reported.

Meanwhile, four more southern African countries have been added to the travel 'red list' from 4am on Sunday. They are Angola, Mozambique, Malawi and Zambia.

"We've been always very clear that we won't hesitate to take further action if that is what is required," Javid said.

"If anyone has travelled to any of these four countries, or any of the other recently red-listed countries in the last 10 days, then they must self-isolate and take PCR tests."

Omicron is considered potentially more dangerous as it has almost twice as many mutations as the Delta variant. However, experts say how much of a concern is still under investigation.

(With input from agencies)