Rich countries should not be ordering booster shots for their vaccinated populations while other countries have yet to receive Covid-19 vaccines, the World Health Organisation said on Monday.
WHO Director General Dr Tedros Adhanom Ghebreyesus said the number of deaths from coronavirus infection was rising again.
Dr Tedros said the Delta variant was becoming dominant and many countries had yet to receive enough vaccine doses to protect their health workers.
"The Delta variant is ripping around the world at a scorching pace, driving a new spike in Covid-19 cases and death," he said.
Dr Tedros said the highly contagious variant, first detected in India, had now been found in more than 104 countries.
"The global gap in Covid-19 vaccine supply is hugely uneven and inequitable," he said.
"Some countries and regions are actually ordering millions of booster doses before other countries have had supplies to vaccinate their health workers and most vulnerable."
He singled out vaccine makers Pfizer and Moderna as companies that were aiming to provide booster shots in countries where there were already high levels of vaccination.
Dr Tedros said they should instead direct their doses to Covax, the vaccine-sharing programme mainly for middle-income and poorer countries.
The WHO's chief scientist, Soumya Swaminathan, said the global health body had so far not seen evidence showing that booster shots were necessary for those who had received a full course of vaccines.
Dr Swaminathan said that while they might be necessary one day, there was no evidence that they were needed yet.
"It has to be based on the science and the data, not on individual companies declaring that their vaccines need to be administered as a booster dose," she said.
Mike Ryan, head of the WHO's emergencies programme, said: "Right now, we are condemning hundreds of millions of people to having no protection.
"We will look back in anger and we will look back in shame", if countries use precious doses on booster shots, at a time when vulnerable people are still dying without vaccines elsewhere, Dr Ryan said.
"These are people who want to have their cake and eat it, and then they want to make some more cake and eat it too," he said.