Japan, which says eating whale meat is a cherished cultural tradition, conducts what it calls scientific research whaling but agrees that endangered species must be protected.

The January death of a female Western gray whale, whose carcass was found in a fishing net off northeastern Japan, was the fourth in two years. Three other females died after becoming entangled in fishing nets along Japan's Pacific coast in 2005.

"This rate of loss of females will, if continued, lead to extinction of the population with high probability," the IUCN said in a statement issued last week, noting that it believes there are only 25 to 35 females of reproductive age.

"It is essential that this spate of net entrapments is investigated thoroughly and that remedial action is taken."

A senior official at Japan's Fisheries Agency said his nation was trying to prevent further deaths but there were limits to what it could do, adding that Japan had yet to be contacted directly by the group.

"All local districts have been told to do everything they can to free whales found caught in fishing nets as speedily as possible," Hideki Moronuki said.

"But there are tens of thousands of such nets around Japan and we can't forbid them all since it would have a huge impact on the fishing industry," he added, noting that much about the whales' livelihood remains unknown.

Activists said Tokyo needed to do more.

"The Western gray whales are quite close to extinction," said Nanami Kurasawa, with the Dolphin & Whale Action Network, an environmental group.

"At the very least we'd like the government to consider more detailed surveys of the whales' habits and perhaps seasonal removal of nets when they're passing by," she added.

From Tuesday, Japan will host a three-day meeting that it says is aimed at "normalising" the International Whaling Commission (IWC), the only global body devoted to whales.

Tokyo says it hopes the unofficial meeting, which is being boycotted by 26 anti-whaling nations, will establish new dialogue within the polarised group, but its ultimate goal is for the IWC to lift a ban on commercial whaling.