Dr Zafia Anklesaria, co-director of the intensive care unit at CommonSpirit's Dignity Health California Hospital Medical Center, attends to a COVID-19 patient in Los Angeles, California, US, on May 18, 2020. [Photo/Agencies]

Coronavirus infections are surging across large parts of the United States, the country's top infectious diseases expert said in a bleak assessment.

The comment highlights how far away the US remains from ending the outbreak, six months into a global crisis that has claimed nearly 500,000 lives and devastated economies.

Even in Europe, which has been loosening travel restrictions following a brutal few months when it was the epicenter of the pandemic, there have been major setbacks.

Germany on Tuesday reimposed lockdowns on more than 600,000 people following a cluster of infections at an abattoir, while the men's tennis world No 1 Novak Djokovic tested positive after hosting an exhibition tournament in the Balkans.

In the US, White House adviser Anthony Fauci warned the next two weeks would be "critical to our ability to address ... surgings" in Florida, Texas and other states.

In response to questioning at a congressional hearing on Tuesday if he was asked about US President Donald Trump's decision last month to withdraw the US from the World Health Organization, Fauci said: "I was not specifically consulted about the withdrawal, or the attempt to withdraw."

Asked whether he has any concerns about the president's plans, Fauci replied: "Yes, I do."

Fauci was joined by Centers for Disease Control and Prevention Director Robert Redfield, Food and Drug Administration chief Stephen Hahn and the head of the US Public Health Service, Admiral Brett Giroir, at the hearing, which lasted more than five hours.

Redfield also said he was "not directly" consulted on Trump's decision to terminate relations with the WHO. He said the CDC has a long history of working with the global health agency and will continue to collaborate.

Worst-hit nation

The country has already recorded more deaths than any other nation, with nearly 800 more fatalities on Tuesday taking its toll past 121,000.

But Trump is determined to fast-track efforts to restore normality.

He continued to stoke controversy on Tuesday, doubling down on weekend comments that he wanted to slow down the testing because so many confirmed infections made the US look bad.

"I don't kid," Trump said, after a White House official described his initial comments as just a joke.

With so many parts of the US unable to contain the pandemic, the European Union was considering blocking US travelers as it reopens its borders to tourism, The New York Times reported.

Agencies, Zhao Huanxin in Washington and Ai Heping in New York contributed to this story.