Trump, speaking to reporters at the White House, said more than one million Americans had been tested for the coronavirus, which he called a milestone.

The president announced on Sunday that the recommendations, which include encouraging people not go gather in groups larger than 10 and to avoid dining in restaurants or bars, would be through the end of next month after initially being put in place for 15 days to curb the virus's spread.

"The guidelines will be very much as they are, maybe even toughened up a little bit," he told reporters on Monday.

Trump, who has faced criticism for playing down the pandemic in its early stages, urged everyone to follow the restrictions.

"Every one of us has a role to play in winning this war. Every citizen, family, and business can make the difference in stopping the virus. This is our shared patriotic duty," he said.

Anthony Fauci, the head of the National Institute of Allergy and Infectious Diseases, said he expected a coronavirus outbreak in the autumn as well but he said the nation would be better prepared to respond.

Trump said he had just spoken with Italian Prime Minister Giuseppe Conte and that the United States would send Italy about $US100 million ($A162 million) worth of medical supplies that are not needed in the United States.

Trump lauded an announcement from Ford Motor Co and General Electric's healthcare unit that they would produce 50,000 ventilators in 100 days.

He also noted General Motors and other US-based companies would make ventilators as well.

Australian Associated Press