The UN's specialist agency in endangered species withheld export quotas for countries such as Azerbaijan, Russia, Iran and Kazakhstan, which last year were able to ship 105 tonnes of caviar and other sturgeon products to the international market.
The UN Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) said it is worried that quota requests for this year, which it did not detail, might be too high in view of falling fish reserves.
"Countries wishing to export sturgeon products ... must demonstrate that their proposed catch and export quotas reflect current population trends," said CITES Secretary-General Willem Wijnstekers.
"To do this they must also make full allowance for the amount of fish caught illegally," he added in a statement.
Sturgeon stock plunge
Environmentalists estimate that Caspian Sea stocks of sturgeon, whose eggs make caviar, have plunged around 90 per cent since the late 1970s due to overfishing.
They welcomed CITES' move, the latest in a series of steps to regulate caviar trade from the big producing areas, including the Heilongjiang/Amur River on the Sino-Russian border.
CITES' 169 member states have the power to ban any state that flouts the rules from trading in an endangered species.
"Sturgeon have been in dire straits for some time and it has been clear that something drastic had to be done," Dr Susan Lieberman, director of WWF's Global Species Program said in a statement.
However, the ban would not affect caviar sales and consumption within Russia, a WWF official said.
The CITES' decision "was foreseen, but unfortunately it does not resolve the problem," said Yevgeny Shvarts of WWF Russia.
"Most consumption takes place in Russia," he said.
But spokeswoman for the Caviar Emptor Coalition, Ellen Pikitch, said: "We think it is great news. It is long overdue".
Officials said quotas sought for 2006 were below those of 2005, which in turn fell from the 147 tonnes granted in 2004.
It is not known how much caviar was actually exported in 2005.
Export quota
"The export quota has been coming down, but so too has the population of the species," CITES chief scientist David Morgan said.
The agency said it needs a clearer idea of how the quota requests were reached before giving the go ahead to further exports to major markets in the European Union and the United States.
It also called on importing countries to do more to ensure that all caviar, which retails at 2,000 to 6,000 euros ($A3,230 to $9,695) a kilogram, had been bought legally.
"If someone finds caviar that it is too cheap, then it is probably illegal," said Ms Pikitch.
Industry officials say that the black market in caviar is about the same as the legal market at some 100 tonnes a year.
CITES' member states have set strict conditions for caviar exports, including a prior accord between producers on amounts, a stock assessment and management plan.
From the start of 2006, sturgeon products must be caught within the same calendar year they are sold, so carryover stock from 2005 cannot be exported as a way of getting round the absence of quotas for 2006.
As the delicacy has a shelf-life of a few months, the impact on consumers may not be immediate, if importers have stocks.