Millions of Americans are on the move during the busy travel days before Christmas, even as Covid infections with Omicron surpass the peak of the Delta wave and hospitals run out of space for patients.

Several thousand travelers were to face a grim Friday with major carrier United saying it was cancelling about 120 flights on Christmas Eve as the "nationwide spike in Omicron cases" impacts flight crews and other operations.

"As a result, we've unfortunately had to cancel some flights and are notifying impacted customers in advance of them coming to the airport," United Airlines media relations office said Thursday in a statement to AFP.

A Christmastime testing crunch meanwhile compounded the country's problems, with pharmacy appointments in big cities all booked, government sites overwhelmed and home kits nowhere to be found.

President Joe Biden -- who as a candidate blasted his predecessor Donald Trump for failures on the same issue -- promised this week to stand up more testing sites and ship out half a billion home kits, beginning January.

At a newly opened federal testing site in New York City's Travers Park, people formed long lines, wearing puffy winter gear to protect against the bone-chilling weather.

"I was planning to meet up with my family, but I might be positive for Covid, so that's something that I don't think is going to be happening," Queens resident Maria Felix said, as she awaited her result.

Government workers also handed out home tests to passersby on the street -- but with only 2,000 set aside for each of the five boroughs in a city of 8.4 million, the items are set to remain scarce for some time to come.

"It is so sad that only 2,000 tests are available," said resident Jocelyn Antigua, who wanted to be sure of her Covid status before meeting her elderly parents.

There were some signs though that the testing holdups were not massively deterring travel. American Airlines, for instance, said it was operating 5,000 daily flights between December 19 and January 1, representing 86 percent of its schedule compared to pre-Covid 2019.

The American Automobile Association estimated 109 million people -- a 34 percent increase on 2020 -- will hit the road, board airplanes or take other transport on trips 50 miles (80 kilometers) or longer between December 23 and January 2.

- Omicron passes Delta -

The festivities are expected to further drive up Covid cases as the heavily mutated Omicron variant pushed the nation's stretched hospitals -- and exhausted health workers -- to the brink.

The strain now accounts for more than 90 percent of all cases in some regions. 

According to Covid Act Now, the seven-day average of new daily cases is running at 171,000 -- about to pass the Delta peak seen in September. 

Intensive care units are running at more than 90 percent capacity in many parts of the country.

"There are more people in the hospitals this year, at this time of the year, than there were last year," John Carney, governor of Biden's home state of Delaware, said in a briefing, where he announced all elective surgeries will be postponed.

Hospitals in Rhode Island and Massachusetts have found it hard to retain health workers, with many leaving due to burnout, Steve Walsh, chief executive of the Massachusetts Health and Hospital Association, told Boston.com.

The Centers for Disease Control and Prevention (CDC) announced new guidance to overcome a shortage in health workers, allowing asymptomatic staff who test positive for Covid to return to work after seven days isolation, down from the previous 10 days.

They also do not need to quarantine if they are exposed to a case, so long as they are vaccinated and boosted.

In New York, Mayor Bill de Blasio announced there would be scaled back New Year's Eve celebrations at Times Square, with the event fully outdoors, masked, and attendees required to show proof of vaccination.

Absent adequate testing, American health authorities are banking on high levels of vaccinations to moderate the number of severe Covid cases -- and vaccination numbers have been strong all week.

On Wednesday, 1.86 million doses were administered, including 1.3 million boosters, White House official Cyrus Shahpar tweeted.

The Food and Drug Administration meanwhile authorized a Covid capsule, developed by Merck, as a treatment for high-risk adults, after greenlighting Pfizer's more effective pill a day earlier.

The two oral treatments are intended to complement vaccines, and help relieve some of the burden of severe cases. 

- Johnson’s plea to Britons -

British Prime Minister Boris Johnson in his Christmas Eve message on Friday exhorted the UK public to get jabbed as a "wonderful" gift for the nation as cases soar.

Johnson said that while little time remained to buy gifts, "there is still a wonderful thing you can give your family and the whole country... and that is to get that jab, whether it is your first or second, or your booster".

The prime minister has rejected harsher virus restrictions in England over Christmas despite a record surge in cases driven by the Omicron variant.

He has focused instead on a drive to offer booster jabs to all adults by the end of the year.

He acknowledged that "after two years of this pandemic, I can't say that we are through it", as the UK set a new record on Thursday of almost 120,000 cases in 24 hours.

Last year Johnson imposed a stay-at-home order on December 19 for London and southeast England that meant millions had to change Christmas plans. 

This year he opted not to tighten rules, saying in his message that "for millions of families up and down the country, I hope and believe that this Christmas is, and will be, significantly better than the last".

People must test themselves before meeting vulnerable relatives, he cautioned, while encouraging them to enjoy a jolly family Christmas.

If the pile of wrapping paper is bigger than last year due to more family members attending, it's because of the vaccine rollout, he stressed.

He also said that getting a vaccine accorded with the teaching of Jesus Christ that "we should love our neighbours as we love ourselves".

Johnson's popularity has been battered by reports of parties held by Downing Street and other government departments during lockdown periods, and his party lost a by-election in a previously safe seat this month.

Some within his own party have also revolted against the most recent Covid measures, particularly on mandatory showing of Covid passes to enter crowded venues such as nightclubs, which they see as violating personal freedoms.

Almost 100 Tory MPs opposed the measure, which was passed by parliament as a whole. 

Johnson's Brexit project is also still mired in wrangles over French fishing rights, a year after the UK reached an agreement with the European Union on a trade deal.