US President Joe Biden's administration will provide 500 million doses of the Pfizer Covid-19 vaccine to about 100 countries over the next two years, US media report.
About 200 million doses will be distributed this year, with the remainder delivered in 2022.
The US has been under pressure to raise vaccination rates in poor countries.
The move comes as Mr Biden landed in the UK at the start of his first foreign trip as US president.
The White House is yet to officially comment on the plan to roll out 500 million doses worldwide.
But when asked if the US government had a vaccination strategy for the world, Mr Biden said: "I have one, and I'll be announcing it," before boarding Air Force One.
An eager crowd awaited his arrival on Wednesday at the RAF Mildenhall airbase in Suffolk, keen to catch a glimpse of Air Force One when Mr Biden touched down on Wednesday evening.
Mr Biden addressed US troops stationed at the base, paying tribute to military families and remembering his late son, Beau, who served in the National Guard.
He praised US allies - "like-minded nations" - and added: "The United States is back!"
"Democracies of the world are standing together to tackle the toughest challenges and the issues that matter most to our future," he said.
Mr Biden later travelled on to Cornwall. The US president and Prime Minister Boris Johnson will meet for the first time on Thursday, ahead of the G7 summit in the south-western corner of England.
Also in his eight-day trip Mr Biden will meet the Queen at Windsor Castle and join his first Nato summit as president. He has said the visit is about "America rallying the world's democracies".
The US president is due to meet his Russian counterpart, Vladimir Putin, at the end of his travels in Europe.
What do we know about the 500 million jabs?
The US government will donate the vaccines to 92 lower-income countries and the African Union, Reuters news agency reports.
Distribution will go through the Covax scheme, which was set up last year to try to ensure fair access to vaccines among rich and poor nations.
The US had previously said it would donate 60m vaccine doses through Covax.
People familiar with the deal told the New York Times that the US would pay for the doses at a "not-for-profit" price.
Albert Bourla, chief executive of Pfizer, is expected to appear with Mr Biden when he officially announces the initiative on Thursday.
Earlier this week, Mr Johnson said he would urge G7 leaders to commit to vaccinating the world against Covid-19 by the end of next year.
The UK and Canada have not yet said how many doses they will provide to the Covax scheme.
What can we expect from the G7 summit?
The G7 (Group of Seven) is made up of the world's seven largest so-called advanced economies - Canada, France, Germany, Italy, Japan, the UK and the US - plus the EU.
Other G7 leaders will arrive in Cornwall on Friday, and meetings will get under way the following morning.
The main topic of conversation will be Covid recovery, including "a stronger global health system that can protect us all from future pandemics".
The agenda also includes climate change and trade.
Mr Biden is expected to warn that the ongoing UK-EU trade row could imperil peace in Northern Ireland. He will call on fellow leaders to protect the gains of the Good Friday agreement.
At the end of the summit, the UK - as the host nation - will publish a document outlining what has been agreed by the leaders.
When will Biden meet Putin?
The first US-Russia summit of the Biden presidency will take place in Geneva, Switzerland, on 16 June.
It comes at the tail end of Mr Biden's trip, giving the president plenty of time to hear from US allies beforehand.
The White House has indicated that he intends to cover a "full range of pressing issues" with his Russian counterpart, including arms control, climate change, Russian military involvement in Ukraine, Russia's cyber-hacking activities and the jailing of Russian dissident Alexei Navalny.
Three organisations linked to Mr Navalny were outlawed by a Moscow court on Wednesday as "extremist".
Mr Putin threw cold water on the Biden meeting last week, saying he hoped to improve the "extremely low level of relations" between the two countries but expected no breakthroughs.
Speaking in Suffolk on Wednesday evening, Mr Biden said that the US was not seeking conflict with Russia but wanted a stable, predictable relationship.
"I've been clear: the United States will respond in a robust and meaningful way if the Russian government engages in harmful activities."
He said he would tell Mr Putin there were consequences for "violating the sovereignty of democracies in the United States, in Europe and elsewhere".
The transatlantic alliance (Nato) would remain a vital source of strength for the UK, Europe and the US, the president said.
The Queen and Nato
The White House said Mr Biden's meeting with Mr Johnson ahead of the summit would "affirm the enduring strength" of the countries' "special relationship".
Both leaders took part in a virtual climate action summit hosted by the US in April but have not yet met in person.
The president and First Lady Jill Biden will also be received by the Queen at Windsor Castle.
It will be the 95-year-old monarch's first meeting with a foreign leader since the death of her husband Prince Philip.
The next leg of Mr Biden's trip takes him to Brussels, where he will meet leaders of other Nato countries.
US relations with Nato were strained under his predecessor Donald Trump, but Nato Secretary General Jens Stoltenberg - in a White House visit on Monday - hailed Mr Biden's "powerful commitment" to US allies.