WASHINGTON (REUTERS) - US President Joe Biden will push world leaders on Wednesday (Sept 22) to do more to curb the Covid-19 pandemic, and announce his country's pledge to buy 500 million more vaccine doses to donate to other nations.

Holding a virtual summit to boost global vaccination rates, Mr Biden hopes to show he is leading by example with the pledge, which will bring US donations to the rest of the world up to more than 1.1 billion doses.

The vaccines from Pfizer and BioNTech will be made in the United States and shipped to low- and lower-middle-income countries starting in January, a US senior administration official said ahead of the announcement.

"This is a huge commitment by the US. In fact for every one shot we have administered in this country to date, we are now donating three shots to other countries," the official told reporters.

A source familiar with the issue said the government would pay some US$7 per dose (S$9.50).

In June, the Biden administration agreed to buy and donate 500 million doses of the vaccine.

Under the terms of that contract, the United States will pay Pfizer and BioNTech around US$3.5 billion or US$7 a dose for the shots.

The US has come under criticism for planning booster shots to fully vaccinated Americans while millions of people around the world still do not have access to the life-saving vaccines.

United Nations Secretary-General Antonio Guterres reprimanded world leaders on Tuesday for the inequitable distribution of Covid-19 vaccines, describing it as an "obscenity" and giving the globe an "F in Ethics".

The virtual summit will focus on vaccinating the world, saving lives by tackling an oxygen shortage and making medicines more available, and preparing for another pandemic in the future, another administration official said.

Targets include getting 70 per cent of countries' populations vaccinated by this time next year.