The World Health Organization on Monday blasted the growing gap between the number of coronavirus vaccines administered in rich and poor countries, branding it a "moral outrage".

The WHO tore into wealthy nations now vaccinating younger people at low risk of developing Covid-19, saying they were doing so at the cost of vulnerable people's lives in low-income countries.

WHO director-general Tedros Adhanom Ghebreyesus said it was "shocking" how little had been done to avert an entirely predictable "catastrophic moral failure" to ensure the equitable distribution of vaccines.

"The gap between the number of vaccines administered in rich countries, and the number of vaccines administered through Covax is growing every single day, and becoming more grotesque every day," he said, referring to the facility that ensures poorer countries can access Covid-19 vaccines.

"Countries that are now vaccinating younger, healthy people at low risk of disease are doing so at the cost of the lives of health workers, older people and other at-risk groups in other countries," Tedros told a press conference.

"The inequitable distribution of vaccines is not just a moral outrage. It's also economically and epidemiologically self-defeating.

"Some countries are racing to vaccinate their entire populations while other countries have nothing."

Tedros said rich countries might be buying some short-term comfort but they were giving themselves a false sense of security.

The UN health agency chief said the more transmission of the virus, the more variants were likely to emerge -- and the more of those that spring up, the more likely they are to evade vaccines.

"As long as the virus continues to circulate anywhere, people will continue to die, trade and travel will continue to be disrupted, and the economic recovery will be further delayed," he said.

The Covax global vaccine-sharing facility has so far distributed more than 31 million doses to 57 countries.