Japan's Olympic wrestler Saori Yoshida (bottom L) and Para-triathlon athlete Mami Tani pose for photos during an opening celemony for the Tokyo 2020 Olympics and Paralympics promotional event "Go for 2020 Street" in Tokyo's Nihonbashi shopping and business district, Japan, September 20, 2016. [Photo/Agencies]

For rowing and canoeing, they suggested three possible sites outside Tokyo, including a city in northeastern Miyagi prefecture, hours north of the capital.

Volleyball and swimming could be covered by renovating existing venues in downtown Tokyo, not far from the site of the Olympic Village, the panel said.

The rowing venue, estimated to cost 6.9 billion yen, now comes in at 49.1 billion yen, the panel noted.

But organizers expressed doubts, saying it might be difficult to win acceptance for changes.

"These sites were chosen over years and approved by the sports federations and International Olympic Committee," said Yoshiro Mori, head of the Tokyo 2020 organizing committee.

"It would be extremely difficult to overturn this."

Tokyo has already shifted venues for several events out of the capital, including sailing and basketball. Cycling will take place in Shizuoka, about 200 km (125 miles) west of Tokyo.

Tokyo organizers have been grappling with a series of blunders.

They were forced to scrap an initial design for the centerpiece National Stadium, site of the opening and closing ceremonies, because it was too expensive, and had to redesign the logo for the games following accusations of plagiarism.