The WHO on Monday that Covid-19 was moving faster than vaccinations, and the G7's vow to share a billion doses with poorer nations was not enough.
Global health leaders also said the pledge was too little, too late, with more than 11 billion shots needed.
Faced with outrage over disparities in vaccine access, the Group of Seven industrialised powers pledged during a weekend summit in Britain to increase their dose donations to more than a billion from 130 million promised in February.
"I welcome the announcement that G7 countries will donate 870 million [more] vaccine doses, primarily through Covax," said the World Health Organisation Director General, Dr Tedros Adhanom Ghebreyesus.
"This is a big help but we need more and we need them faster. Right now, the virus is moving faster than the global distribution of vaccines.
"More than 10,000 people are dying every day. These communities need vaccines and they need them now, not next year."
While people in many wealthy nations are enjoying a return to a sense of normality with high vaccination rates, the inoculations remain scarce in less well-off parts of the world.
In terms of doses administered, the imbalance between the G7 and low-income countries, as defined by the World Bank, is 73 to one.
Many of the donated G7 doses will be filtered through Covax, a global body charged with ensuring equitable vaccine distribution.
Run by the WHO, Gavi, the Vaccine Alliance and the Coalition for Epidemic Preparedness Innovations, it has shipped more than 87 million vaccine doses to 131 countries, which is far fewer than anticipated.
The WHO wants at least 70 per cent of the world's population vaccinated by the next G7 meeting in Germany next year.
"To do that we need 11 billion doses. The G7 and G20 can make this happen," Dr Tedros said.
Doctors Without Borders questioned how sincere the G7 was in pursuing vaccine equity.
"We need to see more clarity around the actual number of doses donated, and exactly how long it's going to take to translate their pledges into real impact and access," the medical charity's Hu Yuanqiong told AFP.
The G7 battle plan also includes commitments to avoid future pandemics, cutting the time taken to develop and licence vaccines to less than 100 days, reinforcing global surveillance and strengthening the WHO.
But observers voiced scepticism at the G7's willingness to follow through on the last point especially.
"I will believe that point when the contributions to WHO are increased," tweeted Ilona Kickbusch, founding director and chair of the Global Health Centre in Geneva.
Others stressed the need to quickly resolve the issue of Covid vaccine intellectual property protection, to boost production.
Negotiations towards a possible suspension of patents for Covid vaccines, and other medical tools needed to battle the pandemic, have just begun at the World Trade Organisation after months of contentious debate.
G7 leaders "say they want to vaccinate the world by the end of next year, but their actions show they care more about protecting the monopolies and patents of pharmaceutical giants", said Max Lawson, Oxfam's head of inequity policy.
Aruna Kashyap, Human Rights Watch's senior counsel for business and human rights, agreed.
"Focusing on vaccines and making charitable donations are not enough," Ms Kashyap told AFP.
"The G7's failure to unequivocally support a temporary waiver of global intellectual property rules is deadly status quo."
The WHO and partners also highlighted the desperate need for funds to overcome the pandemic.
More than $16 billion is still needed this year to fully fund efforts to speed up production and access to Covid-19 diagnostics, treatments and vaccines.
That figure represents less that 1 per cent of annual global defence expenditure, said the WHO's emergencies director, Dr Michael Ryan.
"Surely we can afford 1 per cent of that to save lives and bring this pandemic to an end," Dr Ryan said.