Key Points:

• President Donald Trump taken to a military hospital as he battles "fatigue" and a fever after Covid-19 diagnosis.
• At 74 and obese, Trump is at serious risk from virus.
• Trump has been treated with Regeneron, an antibody cocktail.
• Officials say there are 11 cases related to the setup and planning for Tuesday's presidential debate.
• First lady Melania Trump has also tested positive, son Barron, Ivanka Trump and Jared Kushner have all tested negative.
• All campaign events featuring the president have been postponed.
• Democratic presidential nominee Joe Biden and his running mate Kamala Harris have tested negative.
• It is less than a month until Americans go to the polls after an election campaign marked by Trump's downplaying of the pandemic.
• Trump's diagnosis has sparked fears of a destabilising effect in Washington and around the world — the nation's 'doomsday planes' flew this morning in a show of strength to warn off potential enemies.

President Donald Trump is being taken from the White House to Walter Reed Military Medical Center as he battles Covid-19.

Earlier, the White House doctor said Trump was"fatigued" and had received an experimental treatment for Covid-19.

Sources told CNN the President had a fever since early Friday morning (US Time), as the virus that has killed more than 205,000 Americans spread to the highest reaches of the US government just a month before the presidential election.

Officials say 11 coronavirus cases have been linked to the US presidential debate held in Ohio on Tuesday.

The statement from the White House doctor said Trump received a single 8 gram dose of Regeneron's antibody cocktail in an experimental approach to treating the virus.

"As of this afternoon the President remains fatigued but in good spirits."

The revelation that Trump contracted the virus came in a tweet about 1am after he had returned from an afternoon political fundraiser.

First lady Melania Trump also tested positive, the president said, and several others in the White House have, too, prompting concern that the White House or even Trump himself might have spread the virus further. The Trumps' son Barron, who lives at the White House, tested negative, and both Ivanka Trump and Jared Kushner tested negative on Friday morning, a White House official tells CNN.

• US President Donald Trump tests positive for Covid-19

The First Lady has a cough and headache, the White House doctor said.

All campaign events featuring Trump and his family have now been postponed. Vice President Mike Pence will continue campaigning.

The University of Notre Dame's president says he has tested positive for the coronavirus less than a week after he attended a White House event without wearing a mask.

Reverend John Jenkins sent an email on Friday to university students and staff saying his symptoms are mild and he will continue to work from home. The announcement came after Jenkins this week apologised for not wearing a mask during Saturday's Rose Garden ceremony Saturday for Judge Amy Coney Barrett, Trump's Supreme Court nominee who is a Notre Dame graduate and law professor.

Both Democratic presidential nominee Joe Biden and his running mate Kamala Harris have tested negative, their campaign said. Vice President Mike Pence tested negative for the virus on Friday morning and "remains in good health," his spokesman said.

Trump has spent much of the year downplaying the threat of the virus, rarely wearing a protective mask and urging states and cities to "reopen" and reduce or eliminate shutdown rules.

Trump's Supreme Court nominee, Judge Amy Coney Barrett, who was with him and many others on Saturday and has been on Capitol Hill meeting with lawmakers, also tested negative, the White House said.

Trump's diagnosis was sure to have a destabilising effect in Washington and around the world, raising questions about how far the virus has spread through the highest levels of the US government. Hours before Trump announced he had contracted the virus, the White House said a top aide who had travelled with him during the week had tested positive.

"Tonight, @FLOTUS and I tested positive for COVID-19. We will begin our quarantine and recovery process immediately," Trump tweeted just before 1am "We will get through this TOGETHER!"

While House chief of staff Mark Meadows on Friday tried to assure the public Trump was conducting business as usual, even as he confirmed the White House knew Hope Hicks, the aide, had tested positive before Trump attended a Thursday fundraiser in New Jersey.

"I can tell you in terms of Hope Hicks, we discovered that right as Marine One was taking off yesterday," said Meadows. Several staffers were pulled from the trip, but Trump did not cancel, even after having been exposed to Hicks.

Many White House and senior administration officials were undergoing tests on Friday, but the full scale of the outbreak around the president may not be known for some time as it can take days for an infection to be detectable by a test. Officials with the White House Medical Unit were tracing the president's contacts.

Trump was considering how he might address the nation or otherwise communicate with the American people Friday, an official added.

Trump was last seen by reporters returning to the White House on Thursday evening and did not appear ill. He is 74 years old and clinically obese, putting him at higher risk of serious complications from a virus that has infected more than 7 million people nationwide.

The president's physician said in a memo that Trump and the first lady, who is 50, "are both well at this time" and "plan to remain at home within the White House during their convalescence."

Trump has been trying all year to convince the American public that the worst of the pandemic is behind them. In the best of cases, if he develops few symptoms, which can include fever, cough and breathing trouble, it will likely force him off the campaign trail just weeks before the election and puts his participation in the second presidential debate, scheduled for October 15 in Miami, into doubt.

Trump's handling of the pandemic has already been a major flashpoint in his race against Biden, who spent much of the summer off the campaign trail and at his home in Delaware citing concern about the virus. Biden has since resumed a more active campaign schedule, but with small, socially distanced crowds. He also regularly wears a mask in public, something Trump mocked him for at Tuesday night's debate.

"I don't wear masks like him," Trump said of Biden. "Every time you see him, he's got a mask. He could be speaking 200 feet away from me, and he shows up with the biggest mask I've ever seen."

In a tweet on Friday morning, Biden said he and his wife "send our thoughts to President Trump and First Lady Melania Trump for a swift recovery. We will continue to pray for the health and safety of the president and his family."

World leaders offered the president and first family their best wishes after their diagnosis, as governments used their case as a reminder for their citizens to wear masks and practice social distancing measures.

Trump's announcement came hours after he confirmed that Hicks, one of his most trusted and longest-serving aides, had been diagnosed with the virus on Thursday. Hicks began feeling mild symptoms during the plane ride home from a rally in Minnesota on Wednesday evening, according to an administration official who spoke on condition of anonymity to disclose private information. She was isolated from other passengers aboard the plane, the person said.

Hicks had been with Trump and other senior staff aboard Marine One and Air Force One en route to that rally and had accompanied the president to Tuesday's presidential debate in Cleveland, along with members of the Trump family. The Trump contingent removed their masks during the debate, in violation of the venue rules.

Multiple White House staffers have previously tested positive for the virus, including Pence's press secretary, Katie Miller, national security adviser Robert O'Brien and one of the president's personal valets. An RNC official confirmed on Friday that chairwoman Ronna McDaniel learned she had tested positive on Wednesday afternoon. She has been at her home in Michigan since last Saturday and did not attend the debate.

But Trump has consistently played down concerns about being personally vulnerable. He has mostly refused to abide by basic public health guidelines — including those issued by his own administration — such as wearing face coverings in public and practicing social distancing. Instead, he has continued to hold campaign rallies that draw thousands of often mask-less supporters.

"I felt no vulnerability whatsoever," he told reporters in May.

Questions remain about why Trump and his aides continued to come to work and travel after Hicks fell ill. Trump travelled to New Jersey on Thursday for the fundraiser, exposing attendees to the virus. Trump's social media director Dan Scavino and press secretary Kayleigh McEnany, who were originally set to join him on the trip, were replaced at the last minute by other aides.

But McEnany still briefed the press on Thursday morning and made no mention of Hicks' suspected illness, raising anew concerns about White House transparency.

It is unclear where the Trumps and Hicks may have caught the virus, but in a Fox interview, Trump seemed to suggest it may have been spread by someone in the military or law enforcement in greetings.

The White House began instituting a daily testing regimen for the president's senior aides after earlier positive cases close to the president. Anyone in close proximity to the president or vice president is also tested every day, including reporters.

Trump is far from the first world leader to test positive for the virus, which previously infected Brazilian President Jair Bolsonaro and British Prime Minister Boris Johnson, who spent a week in the hospital, including three nights in intensive care. Former Italian Prime Minister Silvio Berlusconi was hospitalised last month while fighting what he called a "hellish" case of Covid-19.

- AP