Activists at Greenpeace intensified their environmental criticism of Amazon and Microsoft by posting signage near the Seattle offices of both companies questioning the cleanliness of the companies' clouds.

Officials at Greenpeace have launched an international "Clean our cloud" campaign this week calling on leading tech companies to be more transparent about their renewable energy use and encouraging them to be more environmentally conscious.

Greenpeace officials today hung an 800-square-foot sign in the shape of a cloud near the offices of Microsoft and Amazon that reads "Amazon, Microsoft: How Clean is Your Cloud?" Greenpeace representatives also handed out black balloons and pamphlets near the company offices summarising findings the organisation has published this week regarding the energy policies. The report found, in part, that Amazon and Microsoft each rely heavily on coal and nuclear electricity to power their data centres.

Today's events followed demonstrations the organisation held outside of European headquarters of Apple, Microsoft and Amazon.

"Amazon and Microsoft have some of the brightest, most innovative engineers in the business. They have the potential to power their cloud with green, renewable energy, but are falling behind competitors Google, Facebook and Yahoo in the race to build a truly clean cloud," says Greenpeace International IT analyst Casey Harrell.

Apple has rebuffed claims made by Greenpeace that the company's Maiden facility uses renewable power sources for only 10% of its electricity needs. Apple says 60% of the data centre's power will eventually be delivered from an on-site solar farm and fuel-cell installation.

Greenpeace says other companies that are more environmentally conscious have made public their efforts to use renewable energy resources. Amazon Web Services and Akamai, Greenpeace notes, have been particularly secretive about their energy policies.