Mushrooming outbreaks of the highly contagious Delta variant prompted China and Australia to impose stricter Covid-19 restrictions on Saturday as the WHO urged the world to quickly contain the mutation before it turns into something deadlier and draws out the pandemic.

China's most serious surge of coronavirus infections in months spread to two more areas Saturday -- Fujian province and the sprawling megacity of Chongqing.

More than 200 cases have been linked to a Delta cluster in Nanjing city where nine cleaners at an international airport tested positive, with the outbreak spanning Beijing, Chongqing and five provinces as of Saturday.

The nation where the disease first emerged has rushed to prevent the highly transmissible strain from taking root by putting more than one million people under lockdown and reinstituting mass testing campaigns.

Worldwide, coronavirus infections are once again on the upswing, with the World Health Organization announcing an 80 percent average increase over the past four weeks in five of the health agency's six regions, a jump largely fuelled by the Delta variant.

First detected in India, it has now reached 132 countries and territories.

"Delta is a warning: it's a warning that the virus is evolving but it is also a call to action that we need to move now before more dangerous variants emerge," the WHO's emergencies director Michael Ryan told a press conference.

He stressed that the "game plan" still works, namely physical distancing, wearing masks, hand hygiene and vaccination.

But both high- and low-income countries are struggling to gain the upper hand against Delta, with the vastly unequal sprint for shots leaving plenty of room for variants to wreak havoc and further evolve.

In Australia, where only about 14 percent of the population is jabbed, the third-largest city of Brisbane and other parts of Queensland state were to enter a snap Covid-19 lockdown Saturday as a cluster of the Delta variant bubbled into six new cases.

"The only way to beat the Delta strain is to move quickly, to be fast and to be strong," Queensland's Deputy Premier Steven Miles said while informing millions they will be under three days of strict stay-at-home orders.

'The war has changed'

The race for vaccines to triumph over variants appeared to suffer a blow as the US Centers for Disease Control released an analysis that found fully immunised people with so-called breakthrough infections of the Delta variant can spread the disease as easily as unvaccinated people.

While the jabs remain effective against severe disease and death, the US government agency said in a leaked internal CDC document "the war has changed" as a result of Delta.

An analysis of a superspreading event in the northeastern state of Massachusetts found three-quarters of the people sickened were vaccinated, according to a report the CDC published Friday.

The outbreak related to July 4 festivities, with the latest number of people infected swelling to 900, according to local reports. The findings were used to justify a return to masks for vaccinated people in high-risk areas.

"As a vaccinated person, if you have one of these breakthrough infections, you may have mild symptoms, you may have no symptoms, but based on what we're seeing here you could be contagious to other people," Celine Gounder, an infectious diseases physician and professor at New York University, told AFP.

According to the leaked CDC document, a review of findings from other countries showed that while the original SARS-CoV-2 was as contagious as the common cold, each person with Delta infects on average eight others, making it as transmissible as chickenpox but still less than measles.

Reports from Canada, Scotland and Singapore suggest Delta infections may also be more severe, resulting in more hospitalisations.

Asked if Americans should expect new recommendations from health authorities or new restrictive measures, US President Joe Biden responded, "in all probability," before leaving the White House by helicopter for the weekend.

He did not specify what steps could be taken.