Health officials warned Americans to brace for a pivotal week in the coronavirus pandemic, as the number of new cases around the world jumped by more than 100,000 in a single day for the first time, with a third of them coming from the U.S.

Some of the hardest-hit cities, including New York, Detroit and New Orleans, are expected to see infections peak in the coming days, new models suggested.

The next week will be “the hardest and saddest week of most Americans’ lives,” Surgeon General Jerome Adams said on Fox News Sunday, drawing comparisons with the 1941 attack on Pearl Harbor and the Sept. 11, 2001, terrorist attacks.

More than 1.2 million people have been infected around the globe as of Monday, according to data compiled by Johns Hopkins University. In the U.S., more than 9,600 people have died from Covid-19, the respiratory disease caused by the coronavirus.

In the U.K., British Prime Minister Boris Johnson was hospitalized Sunday after suffering persistent symptoms of Covid-19 10 days after he tested positive for the virus, his office said. It said his admission to hospital, under his doctor’s advice, was a precautionary step.