Senators listen as Dr Anthony Fauci speaks remotely during a virtual Senate Committee for Health hearing. AP

America's top infectious diseases official has warned that ending lockdowns too quickly could trigger uncontrollable new coronavirus outbreaks, as the global death toll closes in on 300,000.

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The stark words from Anthony Fauci came as much of the United States and Europe press ahead with easing curbs that have confined billions to their homes to stem the spread of the disease.

Opening up continues despite the toll spiking in some of the world's most populated countries, with Brazil, Russia and the US all reporting bad news.

Dr Anthony Fauci is seen in a frame grab from a video feed as he testifies remotely during a US Senate Committee. Reuters

Concerns were compounded Tuesday by Fauci's warning to Congress that a run of 14 days with drop in cases was a vital first step before lifting lockdowns.

"If a community or a state or region doesn't go by those guidelines and reopens... the consequences could be really serious," he said.

"There is a real risk that you will trigger an outbreak that you may not be able to control."

Fauci said the true number killed by the epidemic in the US is likely higher than the official toll of over 82,000 -- the world's highest.

On Tuesday the US registered 1,894 coronavirus deaths in 24 hours, a steep rise after daily tolls fell below 1,000 on Sunday and Monday.

The expert's cautionary message stands at odds with rhetoric from President Donald Trump, who has pressed for rapid steps to rekindle the devastated US economy before a November election.

A Senate staffer wears a mask during the Senate Committee hearing on coronavirus in Washington. Reuters

Trump has sought to shift focus onto the role of China, where the virus first emerged in December.

Republican allies in the Senate on Tuesday proposed legislation that would empower the president to impose sanctions on Beijing if it does not give a "full accounting" for the outbreak.

"The Chinese Communist Party must be held accountable for the detrimental role they played in this pandemic," said Senator Jim Inhofe.

"Their outright deception of the origin and spread of the virus cost the world valuable time and lives as it began to spread."

In the House of Representatives meanwhile, Democrats unveiled a $3 trillion virus response package, the largest yet, to fund efforts to fight the pandemic and provide emergency payments to millions of American households.