"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 organization 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 are unaffected.
Officials at the U.N. organization 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 Jan. 15, and denied that the organization'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."
"We're hoping, but the problems run deep," said David Morgan, head of the CITES scientific division.
"The socioeconomic 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.
