President Joe Biden will make the pledge at a virtual Covid-19 summit on the sidelines of the UN General Assembly, US officials have said.
The additional jabs will see the total US commitment on vaccine sharing exceed one billion jabs.
Experts say some 11 billion doses are required to vaccinate at least 70% of the global population.
The World Health Organisation has set a minimum target of 40% vaccine coverage in every country by the end of 2021 but the goal is unlikely to be met.
While many high-income countries have now given at least one shot to more than half their populations, only 2% of people in low-income countries have had their first dose, according to data from the University of Oxford.
Source: BBC News