The department's report was released Friday. It says Canadian tar sands are likely to be developed regardless of U.S. action on the pipeline and other options to get the oil from western Canada to Texas Gulf Coast refineries are worse for the environment.
The latest environmental review stops short of recommending approval of the project.
But the report gives the Obama administration political cover if it chooses to endorse the pipeline in spite of opposition from many Democrats and environmental groups. State Department approval of the project is needed because it crosses a U.S. border. A decision is not expected before the summer.