OSAKA (Kyodo) -- With just one month to go until the World Exposition in Osaka opens, predictions of a nearly 3 trillion yen ($20 billion) economic impact are looking shaky as advance ticket sales remain slow and pavilions face delays in completion.

A visitor target of 28.2 million for the April 13-Oct. 13 event on the artificial island of Yumeshima in Osaka Bay is the basis for the government's estimate of an economic impact worth 2.92 trillion yen across the country.

But advance tickets sold as of March 5 stood at 8.07 million since they went on sale in November 2023, far off the target of 14 million set by the organizers. Meanwhile, only about 20 percent of so-called Type A self-built pavilions, considered the highlight of the expo, have been certified by the organizers as completed.

Hiroyuki Ishige, secretary general of the Japan Association for the 2025 World Exposition, acknowledged that the goal of 14 million advance ticket sales was "extremely ambitious."

"It is true there is a risk that construction (of the pavilions) will not be finished in time for the expo's opening," Ishige said, vowing to provide necessary assistance.

Visitors are required to book tickets online for specific dates and times to minimize queues, but critics say the ticket reservation system is too complex and has contributed to the lackluster presales.

The organizers have recently been forced to review their original plan and introduce same-day tickets.

Rises in materials costs, delays in making contracts with building contractors due to complex designs, and labor shortages are among the factors that have hampered progress in pavilion construction.

The association has requested participating countries to finish constructing the pavilions by the middle of January.

Of the 47 countries building the Type-A pavilion, however, only eight -- Ireland, the Philippines, South Korea, Australia, Bulgaria, the Netherlands, Malaysia and Hungary -- have had their buildings certified by the association as completed.

Foreign participants have also been asked to complete their interior displays by around March 13 and gain the association's approval for accepting visitors. No country has reached that stage yet.

It was only in December last year that all countries planning Type A pavilions started construction work. Some countries originally planning to set up their own pavilions have switched to the simpler Type X pavilions that are built by the organizer and rented out to participants.

The site's construction cost has nearly doubled from the initial estimate to 235 billion yen, with the central government, the Osaka prefectural and municipal governments, and the private sector each bearing a third of the bill.

In a survey by the Osaka prefectural and city governments in December, one-third of respondents in Japan expressed interest in visiting the expo, below the expo association's target of 50 percent.

Japan is hosting the World Exposition for the third time, following events in 1970 in Osaka Prefecture and in 2005 in Aichi Prefecture, central Japan.

In addition to the Japanese government and private sector, 158 countries and regions and seven international organizations are planning exhibitions at pavilions, according to Japanese Foreign Ministry.