WASHINGTON, Sep 22 (AFP) - US President Joe Biden addressed a COVID-19 summit of world leaders on Wednesday (Sep 22) 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.

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

"We need other high income countries to deliver on their own ambitions," he said in his opening remarks. "We're not going to solve this crisis with half measures."

Biden 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 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 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 United Nations as president, Biden told delegates that Washington had put more than US$15 billion towards the global COVID-19 response.