It has since rapidly overtaken Delta as the world's dominant Covid variant because it is more transmissible, though it appears to cause less severe illness.
"In the age of effective vaccines, half a million people dying, it's really something," Mahamud told a live interaction on the WHO's social media channels.
"While everyone was saying Omicron is milder, (they) missed the point that half a million people have died since this was detected," he said.
"It's beyond tragic."
Maria Van Kerkhove, the WHO's technical lead on COVID-19, said the sheer number of known Omicron cases was "astounding," while the true number would be much higher.
"It makes the previous peaks look almost flat," she said.
"We're still in the middle of this pandemic," she said. "Many countries have not passed their peak of Omicron yet."
Van Kerkhove said she was extremely concerned that the number of deaths had increased for several weeks in a row.
"This virus continues to be dangerous," she said.
Growth of Omicron
In their weekly COVID-19 epidemiological update issued later Tuesday, the WHO said nearly 68,000 new deaths were reported last week—up 7% compared to the previous week.
Meanwhile, the number of new weekly Covid cases decreased by 17% to nearly 19.3 million.
The WHO's Europe region accounted for 58% of new confirmed cases last week, and 35% of new deaths. The Americas made up 23% of new cases and 44% of new deaths.
The pandemic is currently "characterized by the continued rapid global spread of the Omicron variant," the report said, with the variant now detected "in almost all countries."
The WHO said Omicron accounted for 96.7% of samples collected in the last 30 days that have been sequenced and uploaded to the GISAID global science initiative. Delta now makes up just 3.3%.
The report said limited data was available for the efficacy of vaccines against Omicron.
"However, available estimates show reduced protection of the primary series COVID-19 vaccines against the Omicron variant for all outcomes (severe disease, symptomatic disease, and infection) than has been observed previously for other variants of concern," it said.
But it added that booster jabs "substantially" improve efficacy.
COVID-19 has killed more than 5.7 million people since it emerged in China in December 2019, according to the report, while over 392 million cases have been recorded.
Nearly 10.25 billion COVID-19 vaccine doses have been administered globally. (AFP)