Unknown assailants have severely beaten and hospitalized an environmental activist in Russia's southwestern city of Krasnodar after he filmed illegal construction of a luxury mansion on the Black Sea coast.

Activist Andrei Rudomakha, head of Environmental Watch of the North Caucasus in southern Russia, suffered skull fractures, a broken nose, and a moderate concussion when he was beaten by three masked men late on December 28.

A colleague posted a photo of Rudomakha after the attack:

Activist Aleksandr Savelyev, who was present at the time of the attack, said Rudomakha and other members of his nongovernmental organization had just returned from the Black Sea coast and were near a house owned by one of the group members when the attack occurred.

"Their goal was to collect the material that we filmed," Savelyev said on December 29, adding that the attackers could not have known where to wait without information provided by police or security services.

"They ran from behind, sprayed [Rudomakha] with pepper spray, then knocked him to the ground and kicked him in the face," Savelyev said.

"They took our backpacks and three cameras" along with laptops and footage of the illegal construction activity, he said.

Savelyev said the residence the group was inspecting is being built without a permit in a coastal forest area near a luxury property that has been linked to Russian President Vladimir Putin.

"They fenced off the forest," Savelyev said. "There is construction equipment, dogs, security guards. It's all very serious."

Savelyev said his group is convinced the new construction is for a senior official in the Russian government.

Krasnodar Krai regional police said they are investigating the attack.

Since 1997, Environmental Watch of the North Caucasus has exposed illegal landfills, the destruction of landscapes, and the contamination of waterways in the regions of Krasnodar Krai, Stavropol Krai, Rostov, Adygea, Karachayevo-Cherkesia, and Kabardino-Balkaria.

Some of their investigations also have exposed illegal land grabs by local Russian officials.

The group has actively campaigned against illegally built villas on the Black Sea and criticized construction in Sochi ahead of the 2014 Winter Olympics as destructive to the environment.

Activists in the group have been attacked in the past. One former member, Yevgeny Vitishko, spent nearly two years in prison on charges of damaging a fence in a forest.

In September 2016, Russia's Justice Ministry declared that the group's activities are political and added the organization to Russia's registry of foreign agents.

With reporting by AFP, AP, and Interfax