The Obama administration gave an important approval yesterday to a controversial pipeline that will pump oil from the tar sands of alberta to the Texas coast.  In a blow to campaigners, who have spent the last week at a sit-in at the White House, the State Department said the proposed 1,700-mile pipeline would not cause significant damage to the environment.  The State Department in its report said the project - which would pipe more than 700,000 barrels a day of tar sands crude to Texas refineries - would not increase greenhouse gas emissions. It also downplayed the risks of an accident from piping highly corrosive tar sands crude across prime american farmland.  Campaigners accused the State Department of consistently overlooking the potential risks of the pipeline.  "The State Department. . . failed to acknowledge the true extent of the project's threats to the climate, to drinking water and to the health of people who would breathe polluted air from refineries processing the dirty tar sands oil," Friends of the Earth said in a statement.  But Kerri-ann Jones, the assistant secretary of state, rejected the charges. She argued that other government agencies had still to sign off on the project.  "This is not the rubber stamp for this project," Jones told reporters, adding that the pipeline would not lead to an increase in greenhouse gas emissions, claiming alberta was going to produce the crude anyway.  "The sense we have is that the oil sands would be developed and there is not going to be any change in greenhouse gas emissions with the pipeline or without the pipeline because these oil sands will be developed anyway," she said.