The White House also plans to announce stricter testing rules for international visitors.
Airlines in the United States were told to hand over the names of passengers arriving from parts of southern Africa hit by Omicron, according to a U.S. Centers for Disease Control and Prevention letter seen by Reuters.
Much remains unknown about the new variant, which was first found on Nov. 8 in South Africa and has spread to at least two dozen countries.
Top U.S. infectious disease expert Anthony Fauci on Wednesday said it could take two weeks or more to gain insight into how easily the variant spreads, the severity of the illness that it causes, and whether it can evade currently available vaccines.
South Africa's National Institute for Communicable Diseases (NICD) said early epidemiological data suggested Omicron was able to evade some immunity, but existing vaccines should still protect against severe disease and death.
World Health Organization (WHO) epidemiologist Maria van Kerkhove told a briefing that data should be available "within days" on Omicron's contagiousness.
BioNTech's CEO said the vaccine it makes in a partnership with Pfizer was likely to offer strong protection against severe disease from Omicron.
Early indications suggesting Omicron may be markedly more contagious than previous variants have rattled financial markets, fearful that new restrictions could choke off a tentative recovery from the economic ravages of the pandemic.
Key sharemarket indices in Japan and Australia were weaker in morning trade on Thursday after Wall Street's major averages fell more than 1% on Wednesday as investors reacted to the first U.S. case and mounting concerns about inflation.
Bank of Japan board member Hitoshi Suzuki said Japan's economic recovery may miss expectations if the spread of the Omicron variant hurts consumption, or supply bottlenecks persist.
"If the impact of supply constraints are bigger or lasts longer than expected, there's a risk economic growth may further undershoot expectations" next year, Suzuki said.
About 56 countries were reportedly implementing travel measures to guard against Omicron as of Nov. 28, the WHO said.
In the latest restrictions, South Korea on Thursday halted quarantine exemptions for fully vaccinated inbound travellers for two weeks as daily coronavirus case numbers rose to a new high. South Korea confirmed its first five cases of the Omicron variant on Wednesday.
The United States has barred nearly all foreigners who have been in one of eight southern African countries.
The European Union brought forward the start of its vaccine rollout for 5-to-11-year-olds by a week to Dec. 13, as the president of the European Union's executive body said there was a "race against time" to stave off the new variant.
"Prepare for the worst, hope for the best," Ursula von der Leyen, president of the European Commission, told a news conference.
Britain and the United States have both expanded their booster programmes in response to the new variant, although the WHO says wealthy countries should instead share more vaccines with vulnerable people in poorer countries where variants are most likely to emerge as long as inoculation rates are low.
(Reporting by Reuters bureaux; Writing Stephen Coates; Editing by Simon Cameron-Moore) ((firstname.lastname@example.org))