Washington is ordering 500 million of Pfizer-BioNTech's Covid-19 vaccines to share with other countries, it was reported on Wednesday.
The move marks a major step towards President Joe Biden's plans for a bigger US role in the fight against the pandemic, which has killed more than 3.7 million people worldwide.
The Washington Post first reported the news shortly after Mr Biden said on Wednesday, before he boarded a flight to the UK, that he would soon have an update on the country's global vaccine strategy.
He is expected to announce it in a speech to the G7 summit as early as Thursday, and Pfizer chief executive Albert Bourla could appear alongside him, The New York Times reported.
The US will send out 200 million doses this year, enough to fully vaccinate up to 100 million people, AP reported.
The remaining 300 million doses will be distributed in early 2022.
More than 52 per cent of the adult population in the US has been fully vaccinated and supply has largely outgrown demand.
The US Centres for Disease Control and Prevention reported a seven-day average of fewer than 850,000 daily vaccinations, down from a high of more than 3 million a day in April.
The Pfizer order helps to solidify Mr Biden's goal of turning the US into the world's vaccine "arsenal".
"Strong American leadership is essential to ending this pandemic now and to strengthening global health security for tomorrow – to better prevent, detect and respond to the next threat," he said this month.
"The United States will be the world’s arsenal of vaccines in our shared fight against this virus."
The Biden administration had already committed to donating 80 million Covid-19 vaccines to some countries by the end of June, including vaccines originally intended for use by Americans.
It is the largest donation made by any country so far.
Tens of thousands of AstraZeneca's vaccines were also "lent" to US neighbours Canada and Mexico earlier in the pandemic.
The World Health Organisation-backed Covax scheme recently announced a shortfall of millions of vaccine doses needed to inoculate people in poorer nations.
WHO chief Dr Tedros Adhanom Ghebreyesus has called on rich countries to help out as the situation becomes a "two-track" pandemic, with infections decreasing in highly vaccinated countries and surging in poor countries with little access to shots.
The doses from the Pfizer-BioNTech order will be sent to the Covax programme and the African Union for distribution in 92 lower-income countries, AP reported.