Russia's prime minister drew worldwide publicity in 2008 when he shot the five-year-old female tiger with a tranquilizer gun and helped place a transmitter around her neck. That allowed visitors to his Web site to follow the animal's prowlings through Russia's wild Far East. A video of the episode is on YouTube.
But the satellite tracking device has been silent since mid-September, which could be due to battery failure, a broken collar or poachers, Vladimir Krever of the World Wildlife Fund said Wednesday.
Tigers are rapidly disappearing from the far-eastern regions of Russian due to poaching and the loss of habitat, conservationists say.
Their number may have declined by 40 per cent since 1997, the Wildlife Conservation Society said in a report released Tuesday, although another major conservation group, the World Wildlife Fund, disputed the figure.
The New-York based Wildlife Conservation Society said only 56 tigers have been spotted in an area of 9,000 square miles (24,000 square kilometers) - about one-sixth of their known habitat in Russia. Based on that, the group estimates the total number remaining in the wild at 300.
A similar estimate in 2005 put the number left in Siberia at 500, a huge increase over the less than 30 that were thought to remain in the 1940s. But the Wildlife Conservation Society said the latest count still shows the animals could face extinction.
"The sobering results are a wake-up call that current conservation efforts are not going far enough to protect Siberian tigers," Dr. Dale Miquelle of the group's Russian Far East Program said in a statement.
The society recommends a greater effort to preserve the tiger's habitat, stronger legal protections and a crackdown on poachers who hunt the animals for hides and bones prized in traditional Chinese medicine.
Krever, of the World Wildlife Fund, disputed the Wildlife Conservation Society report.
"It is absolutely incorrect," Krever told The Associated Press. "There's possibly been a decrease in the last two years, but definitely not 40 per cent."
Krever said deep snow in the last two years limited the tigers' ability to roam, making it harder to count them. His group agreed, however, that the tigers face a loss of habitat.
Sergei Aramilev, of Russia's World Wildlife Fund, said Chinese poachers have begun attaching explosives covered with animal fat to tree branches. When tigers and endangered Amur leopards swallow the bait, he said, it explodes in their mouths.
The World Wildlife Fund's Russian branch has estimated that 30 to 50 Amur tigers are killed every year.
Illegal deforestation in Russia's Far East and corruption among poorly paid park rangers may also be contributing to the tigers' decline, said Sergei Berezniuk of the Fenix Fund, an environmental group in the Pacific coast city of Vladivostok.
Earlier this month, Russian officials and environmentalists said they would hold a "tiger summit" in Vladivostok next September to coordinate multinational efforts to protect tiger populations.
The goal of the program, which could involve as many as 13 countries, would be to double the number of tigers globally to 6,500 by 2022. The total now is believed to be 3,200, according to the World Wildlife Fund.
Weighing up to 600 pounds (272 kilograms), Siberian tigers - also known as Ussuri, Amur or Manchurian tigers - prey on wild boars, deer and bears.
They once roamed most of Eurasia from the Black Sea to Central Asia, but now are limited to the forests of Russia's Far East and the Chinese province of Manchuria. In China the killing of a Siberian tiger is punishable by death.