President Barack Obama says that the proposed Keystone XL pipeline project from Canada to Texas should only be approved if it doesn't worsen carbon pollution.

The pipeline has become a contentious issue, with Republicans touting the jobs it would create and demanding its approval and environmentalists urging the Obama administration to reject it because it would carry oil from Canadian tar sands to the Texas Gulf Coast.

"Allowing the Keystone pipeline to be built requires a finding that doing so would be in our nation's interests," Obama said. "Our national interest would be served only if this project does not significantly exacerbate the problem of carbon pollution."

Obama took that stand Tuesday in a speech on his climate change plan at Georgetown University.

The White House has insisted the State Department is making the decision about whether the pipeline is in the national interest, but Obama made it clear he was instructing the department to approve it only if the project won't increase overall, net emissions of greenhouse gases.

A State Department report on the pipeline earlier this year acknowledged that development of tar sands in Alberta would create greenhouse gases, but also made clear that other methods to transport the oil, including rail, trucks and barges, also pose a risk to the environment. For instance, a scenario that would move the oil on trains to mostly existing pipelines would release 8 percent more greenhouse gases such as carbon dioxide than Keystone XL, the State report said.

Obama's instruction to the State Department relates to overall, net emissions, taking into account methods that would be used to ship the oil if the pipeline weren't built.