The World Health Organization's Director-General Tedros Adhanom on Saturday said it will not be easy to make next year's Tokyo Olympics a safe global gathering after the COVID-19 pandemic.

Speaking at a joint news conference with the IOC, Tedros called for "national unity and global solidarity" in the fight against the disease ahead of the Olympics. The games were slated for 2019 but were postponed as the pandemic ravaged the globe.

The Summer Games opening ceremony is now due on July 23, 2021, after the International Olympic Committee and organizers in Japan used WHO advice in March before agreeing a one-year delay.

"We hope Tokyo will be a place where humanity will gather with triumph against COVID," Tedros said at WHO headquarters.

"It is in our hands, but it is not easy. If we do our best, especially with national unity and global solidarity, I think it's possible," he said.

Some 11,000 athletes from across 200 countries are expected to compete at the Olympic Games. These will be joined by thousands of officials who will be housed at an athletes village complex of 5,600 apartments at Tokyo Bay.

Health experts, including in Japan, have questioned how the 33-sport Olympics can be run before an effective global vaccine program is in place.

"Nobody can at this moment in time really give you a reliable answer on how the world will look like in July 2021," IOC president Thomas Bach acknowledged.

"It is too early to start speculation on different scenarios and what it may need at the time to guarantee this safe environment for all participants."

The COVID-19 pandemic has infected close to 4.6 million people globally, with a death toll exceeding 309,000.