Official global numbers have passed the "tragic milestone" of 4 million COVID-10 fatalities, the World Health Organization (WHO) said, adding that the pandemic's true toll was probably higher.

WHO chief Tedros Adhanom Ghebreyesus announced the landmark had been reached, more than 18 months since the outbreak emerged in China in December 2019.

"The world is at a perilous point in this pandemic. We have just passed the tragic milestone of four million recorded COVID-19 deaths, which likely underestimates the overall toll," Tedros told a press conference at WHO headquarters in Geneva.

The U.N. health agency's director-general said some countries with high vaccination coverage were now "relaxing as though the pandemic is already over", dropping public health measures and planning to roll out booster shots.

But he said that far too many countries all over the world are seeing sharp spikes in cases and hospitalization, due to fast-moving virus variants and a "shocking inequity" in global access to vaccines.

"This is leading to an acute shortage of oxygen, treatments and driving a wave of death in parts of Africa, Asia and Latin America," Tedros said.

"Vaccine nationalism, where a handful of nations have taken the lion's share, is morally indefensible."

At this stage in the pandemic, the fact that millions of health and care workers have still not been vaccinated is abhorrent."

Tedros said variants were currently outpacing vaccines due to the inequitable distribution of available doses, which he said was also threatening the global economic recovery from the COVID-19 crisis.

"From a moral, epidemiological or economic point view, now is the time for the world to come together to tackle this pandemic collectively."