Russian prosecutors have warned one of the country's leading opposition parties that it could face charges of extremist activity under laws Kremlin critics say are aimed at stifling dissent.

Prosecutors in the Krasnodar region of southern Russia issued the warning to a local office of the Yabloko Party for helping a political analyst publish several books highly critical of Russian President Vladimir Putin. The warning involved several books by analyst Andrei Piontkowsky, including Unloved Country, which sharply criticizes the Russian leader's policies.

Yabloko Party officials are expressing concern that the action could be a sign of increasing official pressure on the opposition.

Mr. Putin last July signed legislation under which a statement criticizing a government official can be treated as extremist if a court rules it constitutes slander.

Yabloko has not been previously associated with extremism.  It has only minimal influence after officials barred many of its candidates from running in regional elections earlier this year.

Some information for this report was provided by AFP, AP and Reuters.