"Last week marked the fourth consecutive week of increasing cases of Covid-19 globally," Tedros said at a virtual press conference from Geneva, adding that "after ten weeks of declines, deaths are increasing again."
The virus variant first identified in India in October 2020 is continuing the spread around the world and has now been detected in 104 countries, he said, Xinhua reported.
"The Delta variant is ripping around the world at a scorching pace, driving a new spike in cases and deaths."
Leaders around the world have responded to the new rise in infections in markedly different ways, with countries such as France adopting new restrictions while others easing them. The United Kingdom (UK), for one, still intends to lift all restrictions on July 19.
At Monday's press conference, the WHO chief warned of the danger of easing measures while many countries in the world still lack vaccines for their population.
"The current collective strategy reminds me of a firefighting team taking on a forest blaze," he said. "Hosing down part of it might reduce the flames in one area, but while it's smoldering anywhere, sparks will eventually travel and grow again into a rolling furnace."
Tedros reiterated his call on governments to share vaccines, adding that "the world should battle together to put out this pandemic inferno everywhere."