US President Donald Trump's oldest son has tested positive for Covid-19.

Donald Trump Jr., 42, was diagnosed at the start of this week and has been quarantining at his hunting cabin since the result, his spokesman said in a statement.

"He's been completely asymptomatic so far and is following all medically recommended Covid-19 guidelines," according to the statement.

Don Jr. is the second of the president's children to test positive.

Barron Trump, 14, was also diagnosed last month, but made a swift recovery.

A firebrand speaker, Don Jr. played a major role in his father's presidential campaign.

His partner, Kimberly Guilfoyle, a former Fox News host, tested positive for the disease in July, and also recovered.

The president himself spent three nights in hospital at the beginning of October after being hit by Covid-19.

First Lady Melania Trump also had a bout of the infection.

The virus has infected 11.8 million Americans and killed more than 253,000