Masked men armed with guns, knives and stun grenades have attacked a camp of volunteer firefighters who came to southern Russia to help to put out forest fires.

The environmental group Greenpeace, whose members were at the camp in southern Russia, said Friday the masked men attacked their camp at night, beating up people and damaging their tents and cars parked nearby. Several people have been taken to the hospital, one of them with a broken nose, another with broken ribs.

The attackers were shouting "Go back to America," the activists said.

The ultra-nationalist Cossack militia considers Greenpeace and other foreign NGOs to be Western spies infiltrating Russia. Greenpeace said its members were blocked in the camp Thursday unable to leave after the Cossacks surrounded it.