But he defended the detention saying Coast Guard officers had no way of knowing who they were.
Two members of the group were detained Sept. 18 in their attempt to scale the Arctic platform.
The Coast Guard seized Greenpeace's ship the Arctic Sunrise the next day and towed it with 30 activists aboard to Murmansk, where they are being questioned by investigators considering piracy charges.
The detained activists are from 18 countries, including two from Canada, and a long detention or trials could draw unwelcome international attention to Russia's tough policy against protests.
Putin, speaking Wednesday at a forum on Arctic affairs, was quoted by the Interfax news agency as saying: "I don't know the details of what went on, but it's completely obvious they aren't pirates."
He added, however, that the officers "didn't know who was trying to seize the platform under the guise of Greenpeace. Especially in view of the events in Kenya, really, anything can happen."
It was unclear whether Putin's comments might foreshadow leniency for the activists, who could face 10-15 years in prison if convicted of piracy.
Piracy carries a potential prison sentence of up to 15 years and a fine of 500,000 rubles, or about $16,200 Cdn.
Two Canadians are among those who could find themselves embroiled in the Russian legal system. Paul Ruzycki of Port Colborne, Ont., was serving as the ship's chief mate when it was seized last Thursday. A Canadian from Montreal whose name has not been released was also on board.
The platform, belonging to state natural gas company Gazprom, is the first offshore rig in the Arctic. It was deployed to the vast Prirazlomnoye oil field in the Pechora Sea in 2011, but its launch has been delayed by technological challenges. Gazprom has said it was to start pumping oil this year, but no precise date has been set.
Greenpeace insisted that under international law Russia had no right to board its ship and has no grounds to charge its activists with piracy.Suggest a correction