US President Joe Biden addressed a Covid-19 summit of world leaders early Wednesday afternoon with a promise to donate a "historic" extra 500 million vaccines to countries struggling to overcome 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.

"We've already shipped 160 million of these doses to 100 countries," Biden said. "For every one shot we've administered to date in America, we've now committed to do three shots to the rest of the world."

The new tranche of half a billion vaccines will be from Pfizer and will go to low-income and middle-income countries. The donations are being made "free of charge, no strings attached," a senior administration official said ahead of the meeting.

"For every one shot we have administered in this country to date, we are now donating three shots to other countries," she added.

In his first speech to the UN as president on Tuesday, Biden told delegates that the United States had put more than $15 billion towards the global COVID response and shipped more than 160 million doses to other countries.

The United States and other wealthy countries have been criticized by the World Health Organization 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 the administration official said: "We're proving that you can take care of your own, while helping others as well."

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 immunization campaign.

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

Biden has invited heads of state, leaders of international organizations, private sector philanthropies, and NGOs for the summit, which begins around 11:00 am Eastern Time (1500 GMT).

President to Call for End of Pandemic

"The president will call on world leaders to elevate the global level of ambition to end the COVID pandemic in 2022, and to build back better health security to prevent and prepare for future pandemics," said a second US official, according to an AFP early this morning.

Washington will seek to rally the world around three goals: 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% vaccination before UNGA (United Nations General Assembly) of next year," the official said.

The summit will include sessions chaired by Biden and Vice President Kamala Harris, and will be attended by WHO chief Tedros Adhanom.

While the latest global 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.