US President Donald Trump late on Thursday praised Tokyo's Olympic preparations and their "magnificent" venue just hours after he suggested delaying the Summer Games for up to a year due to the coronavirus.

Trump's apparent change of heart came after a phone call with Japanese Prime Minister Shinzo Abe.

"Just had a great conversation with Prime Minister Abe of Japan. I told him that the just completed Olympic venue is magnificent," Trump tweeted.

"He has done an incredible job, one that will make him very proud. Good things will happen for Japan and their great Prime Minister. Lots of options!"

Just hours earlier Trump was the first foreign leader to suggest delaying the Tokyo Olympics because of the coronavirus, dropping a bombshell on his "good friend" Abe.

"Maybe they postpone it for a year," Trump told reporters in the Oval Office, 19 weeks before the opening ceremony in Tokyo's Olympic Stadium.

Trump's comments came the same day as actresses dressed as ancient Greek priestesses held the ceremonial flame lighting at a ruined temple in the original Olympia, Greece.

It was all the more surprising given Trump's repeated declarations of close friendship with Abe and his reluctance previously to give advice on the situation.

Japanese organisers have reiterated that the coronavirus pandemic will not derail the Games scheduled to run from 24 July to 8 August, even if major sporting events, travel and financial markets are already seeing massive disruption worldwide.

The International Olympic Committee also says there has not been any talk of cancellation or postponement.

"With the lighting of the Olympic flame in Olympia today, the International Olympic Committee confirms its full commitment to the success of the Olympic Games Tokyo 2020," the IOC said in a statement.

"We remain absolutely in line with our Japanese hosts in our commitment to delivering safe Olympic Games in July this year."

Despite those assurances, fear of the easily transmitted virus forced the torch lighting ceremony to take place without spectators in Greece's Peloponnese region.

Trump said he admired Japan's preparations for hosting the Olympics for the first time since 1964, although characteristically he also praised his own record as a real estate developer.

"It's a shame," he said. "I used to be in the real estate business as you probably heard. They built some - and I built beautiful buildings - and they built some really beautiful buildings."

Just had a great conversation with Prime Minister Abe of Japan. I told him that the just completed Olympic venue is magnificent. He has done an incredible job, one that will make him very proud. Good things will happen for Japan and their great Prime Minister. Lots of options!

— Donald J. Trump (@realDonaldTrump) March 13, 2020