Live updates: follow the latest news on Covid-19 variant Omicron
US Defence Secretary Lloyd Austin announced on Sunday that he has tested positive for Covid-19.
Mr Austin said he is suffering mild symptoms and will attend meetings online as he quarantines at home for the next five days.
News of his positive test comes after the Pentagon last week tightened restrictions at its headquarters over concern about the highly transmissible Omicron variant that has led to a sharp increase in Covid-19 infections throughout the world.
The military is faced with the challenge of maintaining troop preparedness, often in close quarters on ships and planes.
Mr Austin, who is fully vaccinated and received a booster in early October, said he last met President Joe Biden on December 21, more than a week before he started experiencing symptoms.
“As my doctor made clear to me, my fully vaccinated status — and the booster I received in early October — have rendered the infection much more mild than it would otherwise have been,” he said.
Mr Austin said he requested a test earlier on Sunday after feeling symptoms while at home on holiday. He was last at the Pentagon on Thursday.
The defence secretary is one of the most senior members of Mr Biden's administration to test positive for Covid-19.
In October, Homeland Security Secretary Alejandro Mayorkas tested positive for the virus.
Death toll rises
US authorities registered at least 346,869 new coronavirus cases on Saturday, with the death toll rising by at least 377 to 828,562, according to a Reuters tally.
Mr Austin said he would retain all authority in running the Defence Department and overseeing military activities around the world.
His deputy, Kathleen Hicks, would represent him in some matters, he said.
“I continue to encourage everyone eligible for a booster shot to get one. This remains a readiness issue,” Mr Austin said.
Roughly 98 per cent of active-duty troops have received at the Covid-19 vaccine, which is now mandatory for them.
Top US infectious disease expert Dr Anthony Fauci said on Sunday there was still a danger of a surge in admissions to hospital due to a large number of coronavirus cases even as early data suggests the Omicron variant is less severe than other mutations.
Omicron was estimated to account for 58.6 per cent of the coronavirus variants circulating in the US as of December 25, data from the US Centres for Disease Control and Prevention shows.
Dr Fauci said the CDC will soon announce a clarification on whether people with Covid-19 should test negative before they can leave isolation, after confusion last week over guidance that would let people leave after five days without symptoms.