US President Joe Biden has signed an order barring most non-US citizens who have recently been in South Africa from entering the United States, effective from Saturday.

Mr Biden's order also reimposes an entry ban, which had been set to expire tomorrow, on nearly all non-US travellers who have been in Ireland, the UK, Brazil and 26 countries in Europe that allow travel across open borders.

Last week, then-US president Donald Trump revoked those restrictions which were imposed last year.

"With the pandemic worsening and more contagious variants spreading, this isn't the time to be lifting restrictions on international travel," White House spokeswoman Jen Psaki said at a news briefing.

South Africa's Foreign Ministry did not immediately comment.

Some health officials are concerned current vaccines may not be effective against the Covid-19 variant first detected in South Africa, which also raises the prospect of re-infection.

The 501Y.V2 variant is 50% more infectious than the regular strain and has been detected in at least 20 countries.

Centers for Disease Control and Prevention director (CDC) head Rochelle Walensky is set to sign a separate order today requiring masks on all airplanes and public transportation for all travellers aged two and older, officials told Reuters yesterday.

The Transportation Security Administration is expected this week to issue a separate security directive related to masks, people briefed on the matter said.

Tomorrow, new CDC rules take effect requiring all international air travellers aged two and older to present a negative coronavirus test taken within three calendar days of travel or proof of recovery from Covid-19 to enter the US.

The CDC will not, as it said on 12 January, grant temporary waivers to airlines to exempt some travellers from countries with limited testing capacity. Numerous US airlines last week sought waivers.

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More than 25 million Covid-19 cases have been recorded in the US since the pandemic began, according to the latest Johns Hopkins University tally.

The milestone was reached only five days after the US, the world's wealthiest and hardest-hit nation, recorded 400,000 deaths from the disease.

Mr Biden has made fighting the coronavirus a priority and is pushing for Congress to approve a $1.9-trillion relief package that would include billions of dollars to boost vaccination rates.

The president, who was inaugurated on 20 January, has said he wants 100 million people vaccinated within his first 100 days in office, and he has called for Americans to wear masks for 100 days.