President Joe Biden will on Thursday announce a "historic" US donation of half a billion Covid-19 vaccine doses for 92 poorer countries, the White House said.

The United States is buying 500 million doses of the Pfizer-BioNTech vaccine, including 200 million set to be delivered around the world by the end of this year. The remainder will be sent out by June 2022, the White House said in a statement.

This "largest ever purchase and donation of vaccines by a single country" will "help supercharge the global fight against the pandemic," the White House said.

Biden, who is attending a G7 summit in Britain this week, "will also call on the world's global democracies to do their parts in contributing to the global supply of safe and effective vaccines."

The United States has come under criticism for sitting on huge stocks of unused vaccines, something the government says was initially necessary as a precautionary measure in its own race to get shots into the arms of Americans.

However, with domestic vaccination rates now having reached 64 percent of adults receiving at least one dose, Washington is moving quickly to reclaim global leadership on the issue.

Dismissing suggestions that the United States is in a so-called vaccine diplomacy contest with Russia and China, the White House describes its initiatives as a return to multilateral action after the nationalist isolationism of Donald Trump's presidency.

This new, huge surge in donations aims "to save lives and end the pandemic," the White House said.

With the pandemic largely tamed at home, "the United States is using the power of our democracy, the ingenuity of American scientists, and the strength of American manufacturing to beat the pandemic globally by helping to vaccinate the world."

The vaccines will be distributed through the international Covax program to "low and lower-middle income nations."

The announcement came on top of an earlier pledge to donate some 80 million vaccines by the end of June, as well as a $2 billion contribution to COVAX funding.

-President Biden has been clear that borders cannot keep this pandemic at bay and has vowed that our nation will be the arsenal of vaccines- the statement said.

Biden hinted at the announcement Wednesday before boarding Air Force One bound for Britain to meet with fellow G7 leaders from Britain, Canada, France, Germany, Italy and Japan.

Asked if he had a vaccine strategy for the world, he said: "I have one and I'll be announcing it."

The New York Times said Biden would appear with Pfizer CEO Albert Bourla to make the announcement.

The deal would see the US paying "not for profit" price for the doses, according to US media reports.