The World Expo in Japan's Osaka has welcomed five million visitors in its first six weeks, organizers say, despite lukewarm enthusiasm for the event ahead of its opening.

A Mars meteorite and a beating artificial heart grown from stem cells are among the displays at Expo 2025, where more than 160 countries, regions and organizations are taking part.

The event opened on April 13 and runs until mid-October, with most of the pavilions encircled by the world's largest wooden architectural structure, a latticed "Grand Ring".

On Monday, the five millionth visitor entered the vast waterfront site, organizers said in a statement.

"The number of visitors to the Expo... increased from four million to five million in seven days," they said, adding that more visitors were coming "with each passing day".

So far 12.4 million tickets have been sold to the event. Organizers have set a total target of 23 million.

Also known as a World's Fair, the Expo phenomenon, which brought the Eiffel Tower to Paris, began with London's 1851 Crystal Palace exhibition.

It is now held every five years in different locations around the globe. 

Osaka last hosted the Expo in 1970 when Japan was booming and its technology the envy of the world. It attracted 64 million people, a record until Shanghai in 2010.

Opinion polls before Expo 2025 opened showed low levels of public enthusiasm, with analysts saying inflation and high accommodation costs due to a record influx of foreign tourists to Japan could put people off buying tickets.