Washington DC: US President Donald Trump, who for months refused to encourage wearing masks to combat the coronavirus, on Monday tweeted a picture of himself with his face covered and touted his patriotism.

"We are united in our effort to defeat the invisible China virus, and many people say that it is patriotic to wear a face mask when you can't socially distance," Trump wrote.

"There is nobody more patriotic than me, your favourite president."

The accompanying picture, in black and white, showed him in a dark mask with the presidential seal.

The photo and call for Americans to unite around masks was a far cry from Trump's mockery of them as symbols of weakness during a pandemic he has repeatedly claimed is overblown.

But his tweet stopped well short of endorsing calls from some for a nationwide order to wear masks in public.

The nation's top medical authorities, backed by several leading Republican figures, say wearing masks is crucial to halting the disease.

Amid a fierce resurgence of the virus in southern and western states, including some Republican strongholds, Trump has come under new pressure to change his attitude.

He wore a mask in public for the first time on July 11.

His Democratic challenger in the November presidential election, Joe Biden, has worn a mask in public for months.