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.
BDST: 1255 HRS, JUN 10, 2021