President Barack Obama signaled Tuesday that the Canada-US Keystone XL oil pipeline will be approved only if it does not increase emissions of the greenhouse gases that contribute to global warming.

"I do want to be clear, allowing the Keystone Pipeline to be built requires a finding that doing so would be in our nation's interest," Obama said in a speech laying out a new strategy to fight climate change.

"Our national interest will be served only if this project does not significantly exacerbate the problem of carbon pollution," Obama said at Georgetown University.

The US State Department is currently preparing its recommendation on whether to approve the project.

It concluded in a draft report in March that the project would have no major impact on the environment. A final decision rests with Obama.

Obama had been widely expected to approve the project, and it was not immediately clear how his remarks on Tuesday would affect that perception.

Supporters of the pipeline, which would carry oil from Canada's tar sands to coastal Texas, insist that it would have a negligible impact on the environment.

Opponents say that Alberta's tar sands are the 'dirtiest' oil on the planet because it must be essentially melted with steaming hot water before it can be refined into useable petroleum products.

The process means more fossil fuels need to be burned as part of the extraction process, which further contributes to climate change.