The United States plans to allow vaccinated visitors to enter the country, a White House official has said.

The US closed its borders to most foreign travellers as the coronavirus pandemic took hold.

Washington is developing "a phased approach that over time will mean, with limited exceptions, that foreign nationals travelling to the United States - from all countries - need to be fully vaccinated," the official said, without specifying a timeframe.

The measures to restrict access to the US were introduced in March 2020 and have had a big impact on many Irish people who live and work there.

Ireland, the EU and the UK have eased their travel restrictions on Americans but the favour has not yet been returned.

Earlier this week, the US Centers for Disease Control issued a Level 4 notice for Ireland, advising that travel here should be avoided.

It stated that "because of the current situation in Ireland, even fully vaccinated travellers may be at risk for getting and spreading Covid-19 variants".

Also today, the United States rejected an appeal from the UN health agency for a moratorium on Covid-19 vaccine booster shots.

The World Health Organization said halting booster shots until at least the end of September would help ease the drastic inequity in dose distribution between rich and poor nations.

"We cannot accept countries that have already used most of the global supply of vaccines using even more of it, while the world's most vulnerable people remain unprotected," said WHO chief Tedros Adhanom Ghebreyesus.

Washington swiftly shot down the proposal.

"We definitely feel that it's a false choice and we can do both," White House press secretary Jen Psaki told reporters, adding that the US has donated more doses than any other country.

The WHO said the moratorium would help towards the goal of vaccinating at least 10% of every country's population by the end of September.

At least 4.27 billion doses have been administered globally so far, according to an AFP count.

WHO vaccines chief Kate O'Brien said there was no convincing evidence yet as to whether booster doses were actually necessary.