Mayor Kitty Piercy of Eugene, Oregon introduced the resolution that supports environmentally sustainable energy not from "dirty" tar sands. It calls for purchasing standards and guidelines to track the life-cycle impact of different fossil fuels.

"In the last few years, U.S. mayors have come out as leaders on environmental initiatives, specifically in the fight to stop global warming," said Susan Casey-Lefkowitz a senior attorney at the Natural Resources Defense Council, NRDC.

Oil extraction from tar sands releases three times as much greenhouse gases as conventional oil and threatens Canada's Boreal forest. Production from the Alberta oilsands is projected to triple from 1.2 million barrels to 3.5 million barrels a day by 2015.

The US Mayors Conference also passed a resolution to phase out use of bottle water in their city operations.