A prominent Russian human rights group says unknown attackers have torched one of its cars in the North Caucasus in yet another assault on the activists.

The group, Memorial, said the car was set on fire late Monday in Makhachkala, the capital of the Dagestan region. Memorial said Tuesday its employees have used it to travel to neighboring Chechnya, where the chief of the local office is in jail on suspicion of drug possession.

Pressure on Memorial mounted earlier this month when Oyub Titiyev, the chief of its branch in Chechnya, was arrested. Memorial has described Titiyev's arrest as an attempt to muffle a rare critic of Chechen leader Ramzan Kadyrov.

Last week Memorial's office in another North Caucasus region, Ingushetia, was torched.