US President Donald Trump and First Lady Melania Trump have tested positive for Covid-19. Photo: AFP

Trump, 74, first announced on Twitter that he and First Lady Melania Trump, 50, had tested positive.

"We will get through this TOGETHER!" he wrote.

The news dropped a political bombshell a month before election day.

Badly behind in the polls against Democratic challenger Joe Biden ahead of November 3, Trump has been using large rallies -- where most people go without masks -- to try to change the subject from his much criticized response to the coronavirus pandemic.

Just Thursday he said in a speech to a New York charity event: "The end of the pandemic is in sight and next year will be one of the greatest years in the history of our country."

But now the leader who has repeatedly cast doubt on the seriousness of the virus -- despite more than 200,000 Americans already dying -- has become the world's highest profile patient, proving that all the resources of the White House could not prevent the risk.

Biden, who has made criticism of the Republican's coronavirus response his key issue, made no immediate statement about the president's health.

World leaders including German Chancellor Angela Merkel wished the president and first lady a speedy recovery, with Russian President Vladimir Putin saying Trump's "vitality, good spirits and optimism" would help him cope.

Technically obese and in his 70s, Trump is in a higher-risk category for coronavirus patients.

Trump's official physician, Sean Conley, said in a statement that the president and his wife "are both well at this time."

"They plan to remain home at the White House during their convalescence," he said, adding he expected the president to continue carrying out his duties.