* Activists charged over protest at Arctic oil platform
* Greenpeace says piracy charges are absurd
* Citizens of 18 countries have been arrested
By Steve Gutterman
MOSCOW, Oct 2 (Reuters) - Russia charged Greenpeace activists with piracy on Wednesday over a demonstration last month against Arctic oil drilling, a charge that could bring long prison terms for a protest in a region the Kremlin sees as a key to future prosperity.
The federal Investigative Committee said authorities had begun charging the 30 people from 18 countries arrested after two Greenpeace activists tried to scale the Prirazlomnaya oil platform, which plays a crucial role in Russia's effort to mine Arctic resources.
By evening, 14 people had been charged with piracy, Greenpeace said, including activists and icebreaking ship crew from Argentina, Britain, Finland, the Netherlands, Poland, Russia and Ukraine, as well as a dual U.S.-Swedish citizen and a British videographer who documented the protest.
Greenpeace said the piracy charge, which carries a jail term of up to 15 years, was absurd.
"It is an extreme and disproportionate charge," said Greenpeace International Executive Director Kumi Naidoo.
"A charge of piracy is being laid against men and women whose only crime is to be possessed of a conscience. This is an outrage and represents nothing less than an assault on the very principle of peaceful protest."
Talking tough, Russian Prime Minister Dmitry Medvedev said concern for the environment did not justify breaking the law.
"Concern for the environment must not be a cloak for illegal actions, no matter how high-minded the principles motivating participants," he said at a meeting on offshore oil extraction in the Caspian Sea in the southern city of Astrakhan.
A court in the northern city of Murmansk, a port city north of the Arctic circle, last week ordered all 30 people who had been aboard the Greenpeace vessel Arctic Sunrise to be held in custody for two months pending further investigation.
The environmental group said the protest at the platform owned by state-controlled energy company Gazprom was peaceful and posed no threat, and that piracy charges have no merit in international or Russian law.
In Washington, State Department deputy spokeswoman Marie Harf confirmed that a U.S. citizen named Dimitri Litvinov was among those charged with piracy. She said Washington understood that a second detained U.S. citizen has not been charged.
Harf made no comment on the charges. She said that the U.S. consulate in St. Petersburg had met both of the detained U.S. citizens.
Prirazlomnaya, Russia's first offshore oil rig in the Arctic, is slated to start operating by the end of the year and is expected to reach peak production of 6 million tonnes per year (120,000 barrels per day) in 2019.
Greenpeace says scientific evidence shows any oil spill from Prirazlomnaya, in the Pechora Sea, would affect more than 3,000 miles (4,800 km) of Russia's northern coastline.
Russia, whose slowing economy is heavily reliant on income from energy exports, hopes Arctic oil and gas will help fuel future growth.
Putin, who has not ruled out seeking a fourth presidential term in 2018, has described Arctic shipping and development as priorities and last month announced plans to reopen a Soviet-era military base in the region.
Naidoo called Russia's treatment of the protesters "the most serious threat to Greenpeace's peaceful environmental activism" since its ship Rainbow Warrior was bombed and sunk while in port in New Zealand in 1985, when the group was protesting French nuclear testing in the Pacific.
Former Rainbow Warrior captain Peter Willcox, an American, captained the Arctic Sunrise during the protest and was among the 30 people being held in detention in Murmansk.
President Vladimir Putin said last week that the protesters were clearly not pirates, but had violated international law.