President Barack Obama said Tuesday that the proposed Keystone XL pipeline project from Canada to Texas should only be approved if it does not worsen carbon pollution.

The $7 billion 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 carbon-intensive 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 in a speech on climate change at Georgetown University. ``Our national interest would be served only if this project does not significantly exacerbate the problem of carbon pollution.''

While his remarks appeared designed to reassure environmentalist fearful that the pipeline will be approved, they could also indicate an easing of the way for the pipeline, if the carbon standard is met.

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 Tuesday he was instructing the department to approve it only if the project won't increase overall, net emissions of greenhouse gases that contribute to global warming.

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.

The report also said that even without the pipeline, extraction of oil from the tar sands would likely not be affected.

A top aide to House Speaker John Boehner, a Republican, said Obama's comments indicated that the pipeline should be approved.

``The standard the president set today should lead to speedy approval of the Keystone pipeline,'' Boehner spokesman Brendan Buck said. ``Based on the lengthy review by the State Department, construction of the pipeline would not have a significant environmental impact. It's time to sign off on Keystone and put Americans to work.''

Still, environmentalist took heart in Obama's remarks, noting it was the first time the administration had directly linked approval of the 1,700-mile (2,735-kilometer) pipeline to its effect on pollution. The White House has previously resisted efforts by environmental groups to link the Keystone project to broader efforts to curb carbon pollution from power plants.

``Today President Obama set a standard that the Keystone XL pipeline cannot harm the climate if it is to be approved. That will be difficult standard to meet,'' said Daniel J. Weiss, a senior fellow at the liberal leaning Center for American Progress.

``For the first time, the president has set a do-no-climate harm standard before he approves the Keystone XL pipeline,'' Weiss said.

Michael Brune, executive director of the Sierra Club, said the argument that the pipeline will exacerbate climate change is ``very strong and compelling.'' He said he expects Obama to reject the pipeline.

TransCanada, the Calgary-based company that has proposed the pipeline, said in a statement Tuesday it was pleased with Obama's comments setting out criteria for pipeline approval.

``The almost five-year review of the project has already repeatedly found that these criteria are satisfied,'' said Russ Girling, TransCanada's president and chief executive.

``If Keystone XL is not built, it's clear that the oil will move to market by truck, rail and tanker, which will significantly add to global greenhouse gas emissions to move the product,'' Girling said. Pipelines far safer than any other option, he added.

The pipeline would carry 800,000 barrels of oil a day across six states to refineries along the Gulf Coast. A southern leg from Cushing, Oklahoma, to ports near Houston, already has been approved and construction is proceeding.

Supporters say the pipeline would create thousands of jobs, help lower fuel prices and bolster North American energy resources.

Opponents call the project a ``carbon bomb'' that would carry ``dirty oil'' that could trigger global warming. They also worry about a spill. Converting tar sands into oil uses as much as 15 percent more energy than conventional oil production.

Obama has twice thwarted the pipeline project amid concerns about a proposed route through environmentally sensitive land in Nebraska. The state's governor and congressional delegation _ all Republicans _ have either backed the plan or relaxed their opposition after the project was re-routed last year.

A national poll by the Pew Research Center showed that 66 percent of those polled favor building the pipeline, compared with 23 percent who oppose it. (AP)