"The Delta variant is well on its way to becoming the dominant variant globally because of its increased transmissibility," Swaminathan told a news conference.
Britain has reported a steep rise in infections with the Delta variant, while Germany's top public health official predicted it would rapidly become the dominant variant there despite rising vaccination rates.
The Kremlin blamed a surge in COVID-19 cases on reluctance to have vaccinations and "nihilism" after record new infections in Moscow, mostly with the new Delta variant, fanned fears of a third wave.
Recent research from the UK suggests the Delta variant may be up to 60% more transmissible than the Alpha variant, which originally emerged in the UK in 2020 and has since become dominant in the U.S. and many other countries.
As of Thursday, more than 350 doctors and medical workers have caught COVID-19 in Indonesia despite being vaccinated with Sinovac and dozens have been hospitalized, officials said, as concerns grow about the efficacy of some vaccines against more infectious variants.
"The Delta variant is now becoming more prominent or more dominant in certain parts of Jakarta and other parts of Java," said Mike Ryan, the head of WHO's emergencies programme.
The district of Kudus in central Java, which has about 5,000 healthcare workers, is battling an outbreak believed to be driven by the more transmissible Delta variant, which has raised its bed occupancy rates above 90%.