WASHINGTON (REUTERS) - US President Joe Biden on Thursday (Dec 2) laid out his strategy to fight the Omicron and Delta coronavirus variants over the winter, including free and insurer-funded at-home Covid-19 testing and new requirements for international travellers.

The US government will require private health insurers to reimburse their 150 million customers for 100 per cent of the cost of over-the-counter, at-home Covid-19 tests, administration officials said, and make 50 million more tests available free through rural clinics and health centres for the uninsured.

Reimbursement for tests, however, will not kick in until January, missing the crucial holiday period when many families and groups gather indoors.

"We're going to fight this variant with science and speed, not chaos and confusion," Mr Biden said at the National Institutes of Health in Maryland, while warning that infections will rise this winter.

"The actions I'm announcing are ones that all Americans can rally behind and should unite us in the fight against Covid-19," he said.

The administration is urging all eligible Americans to get vaccinated or booster shots to fight Omicron, which is spreading quickly around the world, and will increase family vaccination sites and expand availability at pharmacies.

Less than 60 per cent of the US population, or 196 million people, have been fully vaccinated, one of the lowest rates among wealthy nations.

The administration says another 100 million are eligible for boosters. The Centres for Disease Control and Prevention said all vaccinated adults should get a booster jab in the light of waning protection over time and the emergence of Omicron.

The United States also plans to require inbound international passengers to be tested for Covid-19 within one day of departure, regardless of vaccination status. Mask requirements on airplanes, trains and public transport vehicles will be extended to March 18.

The new plan will also improve care for those who get Covid-19, tripling the number of "surge response teams" that provide extra staff at hospitals that are overrun with patients to 60 from its current level, Mr Biden said.

It will speed more medications "recommended by real doctors not conspiracy theorists", he added.

The efforts to expand testing and shots come as the world faces new threats from the Omicron variant, and the US confronts a heavily entrenched, politically fuelled anti-vaccination culture.

Fears about the variant have pounded financial markets and created doubts about the speed of the global economic recovery as the pandemic rages on.

The White House is considering further restrictions and ways to boost testing and vaccinations, press secretary Jen Psaki said.

Ms Psaki told reporters earlier on Thursday that the Biden administration would lay out details on whether private health insurance companies will get government money to reimburse customers for over-the-counter tests when it releases guidance on the issue by Jan 15.

However, a White House official late on Thursday said the government will not reimburse private health insurers for the cost of at-home tests.

Insurers were required to cover diagnostic testing for Covid-19 "without any cost-sharing requirements during the public health emergency," the official said.

Additional free tests at healthcare clinics should be available as soon as this month, Ms Psaki said.

Mr Biden said free tests would be available for pickup at thousands of convenient locations for those without private insurance.

"The bottom line: This winter, you'll be able to test for free in the comfort of your home and have some peace of mind," he said.

The largest US employer-based health insurers include Cigna, UnitedHealth Group and CVS Health. Currently, insurers are reimbursed a set amount by the government for most medically necessary Covid-19 tests performed in labs and medical offices.

Ms Kristine Grow, spokesman for industry lobby group America's Health Insurance Plans, said the industry was working with the administration to make sure the impact of any testing plan was fully understood.

Areas of concern include price gouging on these tests, higher premiums and clear rules and guidance for implementation, she said.

Morningstar analyst Julie Utterback said she views the government's plan as a shift in the potential site of testing, rather than a significant increase in costs for health insurers, assuming at-home tests can be accepted as valid.

"To control the spread of the virus from a policy perspective, I see the logic in trying to keep infected people at home instead of forcing them to interact with people outside their household when they are experiencing symptoms," Ms Utterback said.

Evercore ISI analyst Michael Newshel said the strategy could come at a notable cost for health insurers, with the coverage requirement possibly lasting till the first half of the year.

More than 786,000 people have died from Covid-19 in the US, including 37,000 in November alone.