SANZAR has written to the Japan Rugby Football Union seeking clarity about the players signed to the new Super Rugby franchise amid concerns about the team's participation in the southern hemisphere provincial competition.
Japan beat off a bid from Singapore to enter a team into the expanded 18-side tournament next year but local media have cast doubts on the side's ability to compete.
JRFU general secretary Noriyuki Sakamoto dismissed reports at the weekend that only a handful of players had been signed, saying the team had already met the quota given to them by SANZAR.
But Brendan Morris, acting chief executive of SANZAR, the tournament's governing body, said he had asked for more details.
"We're proposing they compete in the toughest rugby competition in the world so I've written to them asking for clarification of who they've signed," Morris told The Australian newspaper.
Japan are yet to name a coach or even pick a name for the team some six months out from the new Super Rugby season, which will also field an Argentine side for the first time.
Sakamoto told Kyodo News on Saturday that the search for a coach had been delayed by the need to find a new national coach, with Australian Eddie Jones to step down after the World Cup.