"These tactics are not peaceful protests. They jeopardise the safety of the people working on board and the protesters themselves, especially aboard a moving vessel at sea."
Shell was "open to an honest discussion about the challenges and benefits of exploring for energy in the Arctic," the spokeswoman added, "but we cannot condone Greenpeace's unlawful and unsafe tactics."
Greenpeace confirmed that the six activists - including Australian Zoe Lennox, 21, and 31-year-old Aucklander Johno Smith - had abseiled off the drilling rig on Saturday and into inflatable boats, before returning to the Esperanza, which had been stationed nearby.
It said they had left because of worsening weather.
"It's astounding that Shell seems to think it has the right to jeopardise our environment and our economy, without being accountable to society," said Annie Leonard, executive director of Greenpeace USA.
"I thank the climbers for being society's eyes and ears on Shell's rig, letting them know that millions of us are watching their every move, because there is simply no such a thing as 'safe' drilling in the Arctic."
- AAP