Joe Biden's chief spokeswoman, Jen Psaki, said that she has tested positive for Covid-19 for the second time, but that the US President was negative.
The press secretary said she had held two socially-distanced meetings with Mr Biden, who is vaccinated, a day earlier.
However, "the president tested negative today via PCR test," she said in a statement, adding that Mr Biden, 79, is "not considered a close contact" as defined by the top US health agency.
As a result of the test Ms Psaki will not travel with the White House team tomorrow to Europe for NATO and EU summits on the Russian invasion of Ukraine, and to another summit in Poland.
It is the second time she has had to cancel a high-profile foreign trip due to Covid.
She also tested positive in October last year, after deciding not to travel to Europe with Mr Biden when members of her family contracted the virus.
Ms Psaki said she had mild symptoms, for which she credited the Covid-19 vaccine, and said she was working from home.
She planned to return to work in person at the end of a five-day isolation period and a negative test.
Meanwhile the Taoiseach, who tested positive for Covid-19 in Washington last week, is expected to decide tomorrow if he will be able to attend an EU summit in Brussels later this week.
Micheál Martin is free of Covid symptoms and has been staying in Washington since his positive test result.
China locks down city of 9m as Covid spreads
China locked down an industrial city of nine million people overnight and reported more than 4,000 virus cases, as the nation's "zero-Covid" strategy is confronted by an Omicron wave.
Health authorities reported 4,770 new infections across the country, the bulk in the northeastern province of Jilin, as the city of Shenyang in neighbouring Liaoning province was ordered to lock down late last night.
China has moved fast in recent weeks to snuff out virus clusters with a pick-and-mix of hyper-local lockdowns, mass testing and citywide closures. It reported two Covid-19 deaths on Saturday, the first in over a year.
Authorities have warned of the risk posed to growth by persistent lockdowns as the country strives to balance the health crisis with the needs of the world's second biggest economy.
Shenyang, an industrial base home to factories including carmaker BMW, reported 47 new cases today.
Authorities there put all housing compounds under "closed management" and barred residents from leaving without a 48-hour negative test result.
Last week, Chinese President Xi Jinping stressed the need to "minimise the impact" of the pandemic on China's economy, but also urged officials to "stick to" the current zero-Covid approach.
But Beijing's virus playbook has been stretched to the limit by the latest Omicron surge, which has forced authorities to free up hospital beds from mild-symptom patients.
Some cities such as Shanghai have avoided a full lockdown and instead imposed a web of individual building lockdowns, even as new daily asymptomatic infections there spiral into the hundreds.
Jilin provincial officials announced yesterday that the first 10,000 doses of Pfizer's oral Covid drug arrived on Sunday, marking the first time Paxlovid has been used in China.
The province last week imposed strict travel curbs banning locals from leaving their cities and counties, while several cities have already confined residents at home.
The southern tech powerhouse of Shenzhen yesterday announced it would lift its week-long lockdown "in an orderly manner", after having partially eased measures on Friday to minimise the impact of virus shutdowns on factories and ports.
Health officials last week revealed only around half of Chinese aged over 80 have been double-vaccinated, as the spectre of Hong Kong's dire Covid mortality rates -- mainly among the unvaccinated elderly -- hangs over Beijing's decision making.