The global Covid-19 death toll has crossed four million, with the worst of the pandemic only just starting to hit some parts of the Asia-Pacific and cases rising again in the United States.

The more infectious Delta virus variant is accelerating outbreaks, and while some nations have started easing restrictions, the World Health Organization warned the world was at a "perilous point".

With fears growing about the spread of the virus, Japanese Prime Minister Yoshihide Suga announced Thursday that the capital Tokyo will be under a state of emergency throughout the Olympics.

"The more infectious Delta variant now accounts for around 30 percent of cases," Japan's virus response minister Yasutoshi Nishimura had said before the emergency announcement.

The emergency will run until August 22, but it will be far looser than the lockdowns seen in other parts of the world such as Australia.

The government there said Thursday it will rush 300,000 vaccine doses to Sydney as Australia's largest city -- in its third week of lockdown -- struggled to bring a Delta outbreak under control.

South Korea, once considered a coronavirus response model, reported nearly 1,300 new infections on Thursday, the highest since the pandemic began.

The surge has forced South Korean authorities to consider imposing the tightest restrictions, under which all public events would be banned.

Elsewhere in Asia, Vietnam's Ho Chi Minh City has gone into lockdown. The virus threat also forced organisers Thursday to postpone the Southeast Asian Games that were due to be held in Vietnam.

But the worst of the Asian outbreaks is in Indonesia, which has become a global hotspot with death rates rising tenfold in a month to more than 1,000 on Wednesday.

Hospitals in the vast archipelago of 270 million have been pushed to the brink by the flood of coronavirus cases.

'Perilous point'

The WHO announced Wednesday that more than four million people have died from Covid-19, but cautioned that the figure was an underestimate of the true toll.

And while many wealthy nations, spurred by rapid vaccination programmes, have started easing and even entirely eliminating restrictions, the WHO urged "extreme caution".

"The world is at a perilous point in this pandemic," said the UN body's chief Tedros Adhanom Ghebreyesus, accusing rich countries of hoarding vaccines and of acting "as though the pandemic is already over".

The Covid-19 challenge has been further complicated in recent weeks by the rise of the Delta variant, which was first detected in India.

In Brazil, which has the world's second-highest known Covid-19 death toll, authorities said Wednesday that the variant has started spreading rapidly in the country's most populous state Sao Paulo.

"It is already circulating in our midst in people who has no travel history or who have no contact with someone who has been, for example, in India," said Sao Paulo health secretary Jean Gorinchteyn.

"We have to pay special attention."

'We've made it'

The Delta variant has also caused a spike in Covid-19 cases in the United States, which has the world's highest availability of vaccines.

But its once-rapid immunisation campaign has dropped off steeply since April.

The seven-day average of new cases rose 21 percent compared with two weeks ago, Centers for Disease Control data showed Wednesday.

Regions in the Midwest and South with lower vaccination rates are experiencing higher case rates than regions with high vaccination rates such as the Northeast.

Amesh Adalja of the Johns Hopkins Center for Health Security told AFP that the likely trend now is that Covid-19 will be more of a problem in areas where vaccinations are low.

"In other parts of the country, the pandemic is largely going to be something that's managed as more of an ordinary respiratory virus."

One such area is New York City, where on Wednesday a ticker-tape parade honoured the everyday "heroes" who kept the city running through the pandemic.

Between marching bands and under confetti, groups of doctors, caregivers, delivery men, public transport workers and food bank employees and others marched as onlookers cheered.

"It's a trauma we've all kind of gone through," said New York resident Sara Cavolo.

"It really hits home the fact that now we're coming back out of this and reemerging, we've made it. It feels just very good to celebrate."