"Omicron has an unprecedented number of spike mutations, some of which are concerning for their potential impact on the trajectory of the pandemic," the WHO said.
"The overall global risk ...is assessed as very high."
Tedros Adhanom Ghebreyesus, WHO director-general, said Omicron's emergence showed how "perilous and precarious" the situation was.
"Omicron demonstrates just why the world needs a new accord on pandemics," he told the start of an assembly of health ministers expected to launch negotiations on such an agreement.
"Our current system disincentivizes countries from alerting others to threats that will inevitably land on their shores."
The new global deal, expected by May 2024, would cover issues such as sharing of data and genome sequences of emerging viruses, and of any potential vaccines derived from research.
Scientist Richard Hatchett, CEO of the Coalition for Epidemic Preparedness Innovations (CEPI), a foundation that funds vaccine development, said Omicron's emergence had fulfilled predictions that transmission of the virus in areas with low vaccination rates would speed its evolution.
"The inequity that has characterized the global response has now come home to roost," he told the talks, noting that Botswana and South Africa had fully vaccinated less than a quarter of their populations.
Omicron was first reported on Nov. 24 from South Africa, where infections have risen steeply.
It has since spread to more than a dozen countries, many of which have imposed travel restrictions to try to seal themselves off. Japan on Monday joined Israel in saying it would close its borders completely to foreigners.
The WHO reiterated that, pending further advice, countries should use a "risk-based approach to adjust international travel measures", while acknowledging that a rise in coronavirus cases might lead to higher morbidity and mortality rates.
"The impact on vulnerable populations would be substantial, particularly in countries with low vaccination coverage," it added.
In vaccinated persons, meanwhile, "COVID-19 cases and infections are expected ... albeit in a small and predictable proportion".
Overall, there were "considerable uncertainties in the magnitude of immune escape potential of Omicron", and more data was expected in coming weeks.
(Reporting by Stephanie Nebehay; Editing by John Stonestreet) ((Stephanie.Nebehay@thomsonreuters.com; +41 58 306 2161; Reuters Messaging: email@example.com; twitter @StephNebehay))