South Africa estimates Omicron has been in the country about at least a month.
Cases have risen at a rate "not seen since the pandemic started," per its president.
Omicron appears to be dominating new infections in most provinces, he said.
South Africa is reporting a sharp surge in COVID-19 cases as the Omicron variant spreads through the country, President Cyril Ramaphosa said in a statement Monday.
The number of positive cases found increased 400% in the past week alone, he said.
Omicron "appears to be dominating new infections in most provinces," per Ramaphosa, although he did not give specific prevalence rates.
He said the nation was "experiencing a rate of infections that we have not seen since the pandemic started."
South Africa first raised alarm bells about Omicron when it detected the variant and reported it to the World Health Organization last month.
It found traces of the Omicron in a sample collected on November 9.
In the month since the country reported this case, South Africa has seen about a 40-fold increase in cases, from a seven-day rolling average of 275 new daily cases (4.58 per million) on November 9 to 10,628 (177 per million) on December 6, per Our World in Data.
Although cases have risen, it is too early to tell whether Omicron is more transmissible, better at escaping immunity, or causes more or less severe symptoms.
Vaccines have remained the most effective way to prevent infection, severe disease, hospitalization, and death even in the face of the Delta variant, and the CDC has urged people to get vaccinated and get their booster shots.
Almost one in four COVID-19 tests are coming back positive, Ramaphosa said. "Compare this to two weeks ago, when the proportion of positive tests was sitting at around 2%," he said.
Recent data showed an uptick in weekly new hospital admissions since Omicron was first seen, from 547 (9.11 per million) in the week of November 6 to 1,027 (17.11 per million) in the week of November 27.
Some early reports have suggested that Omicron could cause less severe disease than prior variants, though experts have said it is too soon to tell.
Ramaphosa said that hospitalization rates lag infections by about two weeks, meaning it is hard to tell how many of those recently infected would go on to need treatment.
He said that hospitals were preparing for more patients all the same.