At least two people were seriously injured when armed masked men attacked a team of volunteer Greenpeace firefighters and a local environmental group early Friday, activists said.

A little after midnight, eight masked men broke into the camp of volunteers who were three days into an expedition to put out fires in the wetlands of the Kuban river delta in southern Russia's Krasnodar region, Greenpeace said.

Before nightfall, a drone had hovered over the fenced camp, according to Environmental Watch on North Caucasus (EWNC), a group working with Greenpeace in the area.

The attackers cut up the activists' tents, threatened them with a gun and severely beat up two people, said Grigory Kuksin, who coordinates the wildfire prevention programme at Greenpeace Russia.

"They broke the nose of (the head of Greenpeace Russia's protected areas programme) Mikhail Kreindlin, who also has a concussion. Another activist is suspected of having his ribs broken," Kuksin told AFP, adding that they had to be taken to hospital.

"The people also suffered from chemical burns from some substance in glass vials that the attackers threw," he added.

The attackers wrote derogatory slogans on the fence around the property where volunteers pitched their camp, including "Pendosy here", using an insulting term for "Americans", Greenpeace said.

Police had visited the scene and some officers had stayed behind to protect the activists.

"This is the first time in our experience this has happened," said Kuksin, who has trained wildfire prevention volunteers and worked with local authorities in Russia for about a decade.

"We understood that fires are dangerous here, but we didn't know that we need bulletproof vests in addition to firefighting helmets," he said.

Amnesty International condemned the attack, saying that failure to investigate would be "akin to official acquiescence" of the incident.

- Police probes -

Local police said in a statement they had launched three probes -- into death treats, theft and bodily harm -- and were looking for suspects involved in the attack.

Andrei Rudomakha, the head of EWNC, which protects the Kuban river delta and had some activists in the expedition, said the attack had probably been coordinated with the security services.

"This was a professional attack, same as before the (2014 Sochi) Olympic Games, when our office was attacked and one of our cars was trashed," he said.

EWNC said the earlier presence of the drone explained how the attackers knew how to locate the tents and vehicles in the dark.

Rudomakha said his group had for years opposed a local state fish farm, which had unsuccessfully sued EWNC for slander and "sets fire to the reeds" in the delta to boost fish production.

Russia's Kuban river delta is a key bird sanctuary on a major migration route, including for rare species like the Dalmatian pelican, but it is also a major agriculture, fishing and hunting area.