MOSCOW — Masked men carrying pistols, knives and truncheons stormed a Greenpeace camp site and severely beat volunteer firefighters in southern Russian on Friday, the environmentalist group said, leaving one activist with a broken nose and another with fractured ribs.

The reason for the assault wasn’t clear. But Greenpeace Russia has been at odds with Russian officials over a variety of issues, including recent claims that Russia’s regional governments were underreporting the size of local forest fires by as much as 80 percent to portray the situation as under control.

Greenpeace deploys volunteer firefighters and runs fire fighting training in many regions.

The attack capped 24 hours of harassment by apparent patriotic vigilantes in the Krasnodar region, an area near the Black Sea, where Greenpeace firefighters had traveled to help put out local grass fires. They had already been confronted by men who identified themselves as Cossacks, socially conservative, pro-Kremlin activists who claim they are descendants of Czarist-era horsemen.

“The attackers climbed through a fence” surrounding a temporary Greenpeace camp in the region, the head of Greenpeace Russia’s firefighting project, Grigory Kuksin, said in a statement.

He said the attackers had “truncheons, knives, pistols” and also damaged cars and slashed tents.

Greenpeace posted a photo of the bloodstained face of Mikhail Kreindlin, a member of the firefighting team who suffered a broken nose, and said that the activists were seeking a safe place and medical attention.

While fighting forest fires may seem an apolitical task, environmental activism can carry a serious price in Russia.