Hundreds of thousands of people in China were in coronavirus lockdown Friday as the country battled its worst outbreak in months, while the United States intensified vaccination efforts in the face of a Delta variant-fueled surge.

The World Health Organization has warned that the highly transmissible strain, first detected in India, could unleash more Covid-19 outbreaks in a high-risk area from Morocco to Pakistan where vaccination rates are low.

In China, a cluster of infections in Nanjing city linked to airport workers who cleaned a plane from Russia earlier this month had reached the capital Beijing and five provinces by Friday.

Hundreds of thousands of people have been locked down in Jiangsu province, of which Nanjing is the capital, while 41,000 came under stay-at-home orders in Beijing's Changping district.

At least 206 infections across China have been linked to the cluster, and the outbreak is geographically the largest in several months.

It challenges Beijing's aggressive containment efforts which have relied on mass testing, lockdowns and swift contact tracing.

The Delta variant is more transmissible than the pathogens that cause SARS, Ebola and smallpox, and as easily spread as chickenpox, according to an internal US Centers for Disease Control presentation reported by The Washington Post and The New York Times.

It has driven recent surges around the world, especially in the Asia-Pacific region.

The Philippines next week will send more than 13 million people in the national capital region back into lockdown because of a Delta-linked increase, the government said Friday.

The variant has also been linked to around half of new cases in Tokyo.

Japan on Friday extended a virus state of emergency in the capital a week into the Olympics, with the city reported a record number of new cases the day before.

Meanwhile, Australia said Friday it would reopen borders and end lockdowns when vaccination rates reach 80 percent.

'You don't have to die'

The United States ramped up efforts to get people vaccinated in the face of a Delta variant-fueled surge.

With infections and hospitalizations rising, President Joe Biden asked every US federal worker to either declare they are fully vaccinated or wear masks and be tested.

"People are dying -- and will die -- who don't have to die," Biden said Thursday. "If in fact you are unvaccinated, you present a problem -- to yourself, to your family, and to those with whom you work."

The president also said he would ask the Pentagon to consider making the coronavirus vaccine mandatory for active duty military personnel, and asked state and local governments to offer $100 to holdouts who get the shot.

Later, the Pentagon said all its military and civilian personnel would require masks, regular testing and travel restrictions if they are unvaccinated.

The CDC has already asked people in virus hotspots -- including the vaccinated -- to wear masks indoors again.

The moves stop short of a politically sensitive vaccination mandate for federal workers, but mark a dramatic return to restrictions after a rapid vaccination phase.

The surge across America -- which has the highest known Covid-19 death toll in the world -- has left early vaccine adopters angry at those who have so far opted against the shot.

"It's almost like they don't care about the rest of the world," Alethea Reed, a 58-year-old healthcare administrator in Washington, told AFP.

"They're being selfish and self-centered."

Israel offers 3rd shot

Nearly 4.2 million people are known to have died of Covid-19 worldwide so far, and vaccines are widely considered to be the main weapon against the disease.

Israel was an early leader in vaccinations, with around 55 percent of the population fully vaccinated using mRNA two-dose vaccines made by Pfizer-BioNTech and Moderna.

Israeli Prime Minister Naftali Bennett announced that those over the age of 60 would be offered a third booster shot from Sunday.

But while wealthy nations like Israel can offer third doses, many poorer nations are struggling to even provide the first.

More than four billion doses have been administered around the world, according to an AFP tally.

High-income countries gave out an average of 97 shots per 100 inhabitants compared with just 1.6 in low-income nations.