US President Donald Trump has signed an executive order prolonging a ban on US employment permits to the end of the year, including student J1 visas and the H1-B visas widely used in the tech industry.

A senior White House official said the move would free up 525,000 jobs, making a dent in the high unemployment rate caused by the coronavirus pandemic.

"President Trump is focused on getting Americans back to work as quickly as possible," the official said, on condition of anonymity.

The executive order extends and widens the 60-day freeze Mr Trump placed on new work permits for non-US citizens two months ago.

The new order includes H1-B visas provided to 85,000 workers each year with special skills, many of them joining the US technology industry.

It also covers most J visas, common for academics and researchers, and L visas used by companies to shift workers based overseas to their US offices.

Meanwhile, two more staff members of the Trump campaign team who were in Tulsa, Oklahoma, for his rally on Saturday have tested positive for the coronavirus, a spokesman said.

The campaign announced on Saturday, hours before the rally, that six members of the campaign's advance staff had tested positive.

"After another round of testing for campaign staff in Tulsa, two additional members of the advance team tested positive for the coronavirus," spokesman Tim Murtaugh said.

"These staff members attended the rally but were wearing masks during the entire event."

Prior to the event on Saturday, Mr Trump's campaign manager Brad Parscale said there had been more than one million requests to attend.

However, the 19,000-seat BOK Centre arena had many empty seats for the event, and Mr Trump and Vice President Mike Pence cancelled planned speeches to an expected "overflow" area outside.

Mr Trump's campaign advisers had seen the rally as a way to rejuvenate his base and demonstrate support, at a time when a string of opinion polls have shown him trailing his Democratic rival, former vice president Joe Biden.

Oklahoma has reported a surge in new coronavirus cases in recent days, and the state's department of health had warned those planning on attending the event that they faced an increased risk of catching the virus.