WASHINGTON (AFP) - President Joe Biden opened a Covid-19 summit of world leaders on Wednesday (Sept 22) with a promise to donate a "historic" extra 500 million vaccine doses to countries struggling to push back against the pandemic.

"This is an all-hands-on-deck crisis," Biden said. "America will become the arsenal for vaccines as we were the arsenal for democracy in World War II."

The pledge from Biden at the summit, held virtually from the White House, brings the total US commitment of donated vaccines to 1.1 billion - more than the rest of the world combined.

"The US has already shipped 160 million of these doses to 100 countries," the White House said in a statement. "For every one shot we've put in an American arm to date, we are now donating three shots globally."

The new tranche of half a billion vaccines will be from Pfizer and aimed at poorer countries.

Biden was also due to challenge world leaders to vaccinate 70 per cent of every country by September 2022, the White House said.

In his opening remarks, he stressed that the surge of vaccines must only be donated, with no "political" strings attached - a veiled dig at China in particular.

The United States and other wealthy countries have been criticised by the World Health Organisation for their plans to roll out booster shots for elderly and high-risk populations, while much of the world faces a severe shortage in doses.

But a senior US administration official told reporters that Washington is "proving that you can take care of your own, while helping others as well."

On Tuesday, in his first speech to the UN as president, Biden told delegates that the US had put more than US$15 billion (S$20 billion) towards the global Covid-19 response and shipped more than 160 million doses to other countries.

70 per cent target

Despite the development of safe and highly effective vaccines in record-breaking time, huge disparities exist between countries with ample supply and others that have barely begun their immunisation campaign.

Just 3.6 per cent of Africa's eligible population has been inoculated - compared with an average of more than 60 per cent in Western Europe.

The summit - technically held on the sidelines of the UN General Assembly - saw Biden and US Ambassador to the UN Linda Thomas-Greenfield hosting a wide variety of health and foreign leaders.

They included UN Secretary-General Antonio Guterres, World Health Organisation chief Tedros Adhanom Ghebreyesus, and the heads of Britain, Canada, the European Union, Indonesia, and South Africa.

Washington will seek to rally the world around three goals, the administration official said.

These are: increasing vaccine supply; saving lives now by resolving the oxygen crisis and access to testing, medicine and therapeutics; and lastly improving future preparedness.

On vaccines, Biden will set an "ambitious target, which will require all countries to step up, so that every country, including low income and low middle income countries can achieve 70 per cent vaccination before" next year's UN General Assembly, the official said.

While the latest global coronavirus wave peaked in late August, the virus continues to spread rapidly, particularly in the United States, which is officially the worst-hit country.

Some 4.7 million have died since the outbreak began in China in December 2019, according to an AFP tally from official sources.