It is now up to exporting nations to come forward with new proposals if they want to restart the money-spinning commerce.
Every year, the Convention on International Trade in Endangered Species (CITES) asks caviar-producing countries for a quota for the following year's catch.
CITES, a UN organisation grouping 169 countries, said on Tuesday it could not approve the 2006 quotas proposed by major exporting nations, saying they "may not fully reflect the reductions in stocks or make sufficient allowance for illegal fishing".
"Since the CITES system only allows sturgeon products to be exported during the year in which they are harvested and processed, as of now it is not possible to export caviar and other sturgeon products from shared stocks," the organisation said.
It was referring to natural habitats shared between several nations, mainly the five around the Caspian Sea: Azerbaijan, Iran, Kazakhstan, Russia and Turkmenistan. Fish-farmed sturgeon is unaffected.
Officials at the UN organisation said CITES could end the freeze if it received satisfactory new proposals but did not give a timetable.
But an Iranian official said CITES had asked for more details by 15 January, and denied that the organisation's move amounted to a formal ban.
To have its quota approved, according to CITES rules, a government must show that trade is "not detrimental to the long-term survival of the species".
The Amur-Heilongjiang river has
seen a fall in the fish population
David Morgan, head of the CITES scientific division, said: "We are hoping, but the problems run deep.
"The socio-economic conditions on the ground are difficult. The governments have the will to fight illegal fishing, but the temptation is big in relatively poor countries," Morgan added.
CITES first imposed caviar trade controls in 1998, after a decline in sturgeon stocks following the break-up of the Soviet Union.
The end of communist-era restrictions spurred illegal fishing and raised fears among environmentalists that sturgeon would be wiped out.
"The commercial extinction of sturgeon is a certainty and is coming soon" if things continue at the current pace, warned Morgan.
The Caspian is the source of 90% of the world's caviar and was already hit with a temporary ban in 2001. Contraband caviar is likely to represent "several times the quantity sold legally", said Morgan.
Aside from the Caspian, CITES pointed to "serious population declines" of sturgeon in shared fishing grounds in the Black Sea and the lower Danube river, where producers include Romania and Bulgaria, and the Amur-Heilongjiang river, which is shared between Russia and China.
Domestic consumption in Russia
is driving poaching of sturgeons
According to Stephanie Theile of the wildlife trade monitoring network TRAFFIC, "a permanent ban would not be a good thing".
"What we need is sustainable trade in sturgeon products," she said, noting a call last month by Russian authorities for a state monopoly on sturgeon.
"The Russians have acknowledged publicly that they have a big problem on their domestic market," Theile said.
CITES also pointed to importers, saying they "must ensure that all imports are from legal sources, and they must establish registration systems for their domestic processing and repackaging plants and rules for the labelling of repackaged caviar".
"The socio-economic conditions on the ground are difficult. The governments have the will to fight illegal fishing, but the temptation is big in relatively poor countries."
head of CITES' scientific division
Many countries have failed to act, it said. The EU still does not have a new labelling system, despite a 2004 deadline.
"There's probably a lack of awareness from the Europeans about the problems we have with illegal trade," said Theile.
According to authorities in France - Western Europe's top caviar market - a suspected 90% of caviar sold there is contraband.
In September the US said it was suspending imports of highly prized beluga caviar after complaining of inaction by Caspian states.
The WWF conservation group on Tuesday cautioned that international action is not enough, saying that domestic consumption also drives poaching, particularly in Russia.