There are concerns the US president, who wore a mask, may have endangered Secret Service staff inside the car.

White House spokesperson Judd Deere said the trip on Sunday had been “cleared by the medical team as safe”.

Questions remain over the seriousness of Mr Trump’s illness after conflicting statements over the weekend.

Speaking on Fox News, White House chief of staff Mark Meadows said doctors would decide later whether the president could be discharged from Walter Reed hospital, near Washington, on Monday.

Mr Meadows said he had spoken with Mr Trump earlier, and that his condition appeared to have improved overnight. The president was taken to hospital on Friday evening, hours after announcing he had tested positive for the virus.

President Trump’s diagnosis has upended his election campaign, as he faces Democratic challenger Joe Biden on 3 November.

A growing number of people around the president, including his wife Melania, senior aides and Republican senators, have tested positive with the virus.

Covid-19 has infected nearly 7.4 million and killed nearly 210,000 people across the US, according to Johns Hopkins University.

What did the president do?

Mr Trump waved to well-wishers from behind the glass of a sealed car after tweeting that he would pay a “surprise visit” to “patriots” outside the hospital. Inside the car, at least two people could be seen wearing protective gear in the front seats, with Mr Trump sat in the back.

Experts say the president’s short car trip broke public health advice to quarantine when seeking treatment for the virus, and may have put Secret Service agents inside the vehicle at risk of infection.

“That Presidential SUV is not only bulletproof, but hermetically sealed against chemical attack. The risk of Covid-19 transmission inside is as high as it gets outside of medical procedures,” tweeted Dr James Philips, a doctor at the same hospital where the president is being treated.

Those inside the president’s car would now need to quarantine for 14 days, he said.

Democrats have also criticised the trip, with House of Representatives Hakeem Jeffries tweeting: “We need leadership. Not photo ops.”

But the White House’s Judd Deere defended the move, saying “appropriate precautions were taken in the execution of this movement to protect the president and all those supporting it, including PPE [personal protective equipment]”.

Meanwhile, NBC News reports that Melania Trump, who has remained at the White House with mild symptoms, decided against visiting her husband in hospital because of the risks to staff.

“She has Covid,” an unnamed official told NBC on Saturday. “That would expose the agents who would drive her there and the medical staff who would walk her up to him.”