Russian investigators on Wednesday formally charged two of 30 detained Greenpeace campaigners for piracy over an open-sea protest against Arctic oil drilling, an activist said.

"The first two activists have been charged with piracy," Mikhail Kreindlin, a representative of Greenpeace, told AFP. "These activists are from Brazil and Britain."

He did not provide further details. Piracy by an organised group carries a punishment of between 10 and 15 years.

The charges of piracy came despite the fact that President Vladimir Putin had last week said that the activists "of course are not pirates." He however said that they did break the law by protesting close to an oil rig.

One of the charged activists is a British video reporter, Greenpeace said, without providing names. The second is a female from Brazil.

Russian investigators accused the activists of piracy after several of them tried to scale state energy giant Gazprom's Prirazlomnaya oil platform in the northern Barents Sea last month.

The group has denied the charges and accuses Russia of illegally boarding its ship in international waters.

The 30 activists from 18 countries are being held in pre-trial detention centres in the cities of Murmansk and Apatity, which are nearly 2,000 kilometres (1,250 miles) north of Moscow and above the Arctic Circle.

All but four of them are non-Russians from countries including Britain, the United States, Finland and Argentina.