The Delta variant of COVID-19, identified in at least 85 countries, is the most transmissible of the variants identified so far and is spreading rapidly among unvaccinated populations, WHO chief Tedros Adhanom Ghebreyesus has warned.

"I know that globally there is currently a lot of concern about the Delta variant, and the WHO is concerned about it too, Director-General Ghebreyesus said at a WHO press briefing on Friday.

The Delta variant was first identified in India.

Delta is the most transmissible of the variants identified so far, has been identified in at least 85 countries, and is spreading rapidly among unvaccinated populations, he said in Geneva.

He noted with concern that as some countries ease public health and social measures, we are starting to see increases in transmission around the world.

More cases means more hospitalisations, further stretching health workers and health systems, which increases the risk of death, he said.

Tedros, who is Ethiopian, chastised unnamed countries for reluctance to share doses with low-income countries. He compared it to the HIV/AIDS crisis, when some argued that African nations were unable to use complicated treatments.

"I mean that attitude has to be a thing of the past," Tedros said, as reported by Reuters. "The problem now is a supply problem, just give us the vaccines."

"The difference is between the haves and the have nots which is now completely exposing the unfairness of our world - the injustice, the inequality, let's face it," he said.

While pointing out that new COVID-19 variants are expected and will continue to be reported, that's what viruses do, they evolve - but we can prevent the emergence of variants by preventing transmission.

In a strong warning, Dr Maria Van Kerkhove, COVID-19 Technical Lead at the WHO said the Delta variant is a dangerous" virus and is more transmissible than the Alpha variant, which was itself extremely transmissible across Europe and any country that it entered.

The Delta variant is even more transmissible, she said, adding that the WHO is seeing trajectories of incidents that are almost vertical" in a number of countries around the world.

Many European countries are witnessing a decline in cases but there are a lot of events happening across the region, including large sporting or religious events or even backyard barbecues.

"All of these actions have consequences and the Delta variant is spreading readily among people who are unvaccinated," Kerkhove said.

While some countries have high percentages of people who are vaccinated, yet the entire population of those nations is not yet vaccinated and many people have not received their second dose or the full course of dose of the COVID-19 vaccines, she said.

Kerkhove underlined that COVID-19 vaccines are "incredibly effective at preventing severe disease and death, including against the Delta variant.

The virus will continue to evolve. And right now our public health and social measures work, our vaccines work, the diagnostics work and the therapeutics work. But there may be a time where this virus evolves and these countermeasures don't. So we need some kind of movement to pull ourselves together to drive transmission down and keep it down, she said.

Kerkhove warned that events that are large scale and see huge crowds will have consequences. We are already starting to see some consequences of these events with increasing transmission again. The Delta variant will make that epidemic curve exponential, she warned.

She urged people to keep themselves safe and make decisions individually about what they need to do every day. "There's a lot that all of us want to be doing, but there's not a lot that we need to be doing right now, she said.

"It's not for the next couple of months... that we need to be thinking about this. We need to be thinking about it right now, because every single decision that we make, leaders make has consequences, good and bad."

The WHO chief said it's quite simple that more transmission means more variants and less transmission means fewer variants.

"That makes it even more urgent that we use all the tools at our disposal to prevent transmission: the tailored and consistent use of public health and social measures, in combination with equitable vaccination," Ghebreyesus said.

He said this is the reason why WHO has been saying for at least a year that vaccines must be distributed equitably, to protect health workers and the most vulnerable.

This week, WHO had said that the Delta variant, the significantly more transmissible variant of COVID-19, continues to be detected in new countries around the world.

The COVID-19 Weekly Epidemiological Update released on June 22 by WHO said that globally, the variant Alpha has been reported in 170 countries, territories or areas, Beta in 119 countries, Gamma in 71 countries and Delta in 85 countries.

Delta, now reported in 85 countries globally, continues to be reported in new countries across all WHO Regions, 11 of which were newly reported in the past two weeks, the update said.

WHO said the four current Variants of Concern being monitored closely Alpha, Beta, Gamma and Delta - are widespread and have been detected in all WHO regions.

The Delta variant is significantly more transmissible than Alpha variant, and is expected to become a dominant lineage if current trends continue.