Greenpeace says investigators have dropped charges against 16 activists who were detained in Russia's Arctic in September.

Authorities on Tuesday notified one crew member that his criminal case was closed. The remaining 29 activists were summoned to receive similar notifications on Wednesday.

Greenpeace on its official Twitter account on Wednesday listed 16 activists who had their criminal cases closed.

The 30 crew members aboard a Greenpeace ship were detained in September and held in custody for two months before they were released in November pending trial. They were originally charged with piracy, but that was then downgraded to hooliganism.

All non-Russian crew members are expected to have the charges against them dropped and be allowed to leave Russia.