Vladimir Kirillov, head of the state's top environmental watchdog, has criticized the World Wildlife Fund's "Adopt a Tiger" program to protect the endangered Amur tiger, calling the name "unacceptable and illegal" because the animals are federal property.

The program is part of the WWF's campaign to protect Russia's remaining 500 wild tigers from poaching and other threats. The program invites donations of 25,000 rubles ($848) per tiger, giving benefactors certificates in advance.

Prime Minister Vladimir Putin is a strong supporter tigers and has a section on his personal web site devoted to the rare Amur tiger.

In his letter to WWF Russia director Igor Chestin, which was posted Tuesday on the Federal Inspection Service for Natural Resources Use web site, Kirillov blasted the fundraising campaign.

"The animal world on the territory of Russia is state property, while rare and endangered animals … are federal property," the letter says, adding that transactions involving endangered wild animals are only permitted with special licenses given out by relevant government agencies. "Using the term 'adoption' for this program is incorrect and unacceptable," the letter says. Kirillov asked the group to "consult the service's specialists" in the future.

The WWF has not yet received the letter, but the organization has prepared an in-depth legal analysis explaining why the usage of the term "adoption" is appropriate, spokeswoman Maria Vinokurova said Thursday.

The word, in its colloquial sense, signifies the personal involvement of a benefactor in the protection of wild tigers, the organization said in a statement.