There have been emotional scenes in US airports and on road borders as travellers across the world are reunited with loved ones in the US for the first time in nearly two years, after border restrictions were lifted.

A steady stream of Canadian visitors – many retirees heading for the sunny southern states – were among the first to cross the border, when rules lifted for non-US citizens for the first time in 20 months on Monday (local time).

The extraordinary US travel restrictions, first imposed in early 2020 in an attempt to halt the spread of the coronavirus, had barred access to non-US citizens who had been in Britain, the 26 Schengen countries in Europe without border controls, Ireland, China, India, South Africa, Iran and Brazil in the previous 14 days.

Trade group US Travel said the countries accounted for 53 per cent of all overseas visitors to the US in 2019.

The unprecedented ban dealt a huge blow to tourism but also kept friends and families from attending weddings, funerals, or meeting new babies.

“This whole half of my life has been missing for almost two years,” Janet Simoni, who lives in London, Ontario, and whose husband lives on the other side of the US-Canada border near Detroit, Michigan, told Reuters.

She said the couple had missed major milestones, including a graduation and funeral.

From Monday (local time), travellers who can show official proof of vaccination and a recent, negative viral test can fly to the US – and many headed to airports in London, Paris and beyond.

“We went from zero activity to one that is similar to October 2019 levels, so before COVID,” said Jerome Thomann, of Paris-based Jetset Voyages travel agency, which specialises in trips to North America.

There were expected to be few if any empty seats on many international flights on Monday, and passenger volume was expected to remain high in coming weeks.

The reopening of the US to British travellers will help all airlines operating between the two countries, but for British-based trans-Atlantic-focused Virgin Atlantic, it meant “the world”, its chief executive said.

“This is the market that is at the heart of everything that we do,” CEO Shai Weiss said.

Delta said in the six weeks since the US reopening was announced international point-of-sale bookings had leapt 450 per cent on the six weeks before the announcement.

Airlines, which have warned there will likely be long queues at first, will check vaccination documentation for international travellers as they currently do for COVID-19 test results.

Also from Monday, the nearly 3200-kilometre border between Mexico and the US opened. Ahead of the reopening, hundreds of migrants arrived at Mexican border cities such as Tijuana, hoping the reset would make it easier to cross and seek US asylum.

Much further north, Kristy Kennedy, an official at North Country Chamber of Commerce in northern New York, watched a live video feed of the border crossing at Champlain, New York.

“Having seen it just so bare at the border for 20 months, I was curious. … I wanted to log in and see if that relationship was as strong as it always has been,” she said.

“I think the two-hour wait and the two-mile-long (3.2-kilometre) lines of cars spoke volumes.”

At land border crossings, US Customs and Border Protection is asking if travellers have been vaccinated and spot check some documentation.

Children under 18 are exempt from the new vaccine requirements. Non-tourist travellers from nearly 50 countries with nationwide vaccination rates of less than 10 per cent will also be eligible for exemption.

-with AAP