Japan will not comply if a ban on international trade in Atlantic bluefin tuna is imposed, a senior official said after the United States threw its support behind the move ahead of a crucial vote on banning exports of the fish that's prized in Japan for sushi.

"If worse comes to worst, Japan will inevitably have to lodge its reservations," Vice Fishery Minister Masahiko Yamada said.

His comments came a day after the United States threw its support behind the ban on the international trade in Atlantic bluefin tuna, which conservationists say risks extinction if current catch rates continue.

Washington showed its support ahead of a March 13-25 meeting in Qatar, where 175 member countries of the Convention on International Trade in Endangered Species of Wild Fauna and Flora, or CITES, will vote on the ban proposed by Monaco. A ban would require support by two-thirds of member nations.

Bluefin tuna stocks in the Eastern Atlantic and Mediterranean dropped by 60 percent between 1997 and 2007, a result of surging demand as well as illegal and underreported catches.

About 80 percent of all Atlantic bluefin ends up in Japan, where the fish is a key ingredient in sashimi and sushi. Fatty bluefin - called "o-toro" here - can go for as much as 2000 yen (NZ$32) a piece in high-end Tokyo restaurants.

Japan opposes the ban, but Yamada said the country is committed to protecting bluefin species.

Tokyo believes that catch quotas, which have already been cut 40 percent this year to 13,500 tons by another body, the International Commission on the Conservation of Atlantic Tuna, should be sufficient.

Environmentalists say the quotas are widely ignored and are too high anyway.

If a trade ban on the species appears inevitable, Japan may register a reservation, which in practical terms means it could engage in trade with any other nation that also registers a reservation.

"There are countries that support our stance. We will continue to make our best efforts until the very last minute," Yamada said.

Other tuna species, including the Pacific bluefin, bigeye and yellowfin, would not be affected by the ban.

The European Commission has proposed that EU governments commit to the ban, although there still appears to be some division. Greece, Malta, Spain and Italy - which have strong fisherman lobbies - have resisted steps to curtail the hunt for bluefin, but recently France signalled it would support a trade ban if its implementation were delayed.