CALGARY — TransCanada launched a double-barrelled legal salvo Wednesday against the U.S. government over its rejection of the company's proposed Keystone XL pipeline.

The company said it intends to file a claim under Chapter 11 of the North American Free Trade Agreement in response to the decision, which it called arbitrary and unjustified.

The Calgary-based firm said it will be looking to recover US$15 billion in costs and damages as a result of what it says is a breach of NAFTA obligations.

TransCanada says it has also filed a lawsuit in the U.S. Federal Court in Texas asserting that President Barack Obama's decision in November to deny construction of Keystone XL exceeded his power under the U.S. Constitution.

The 1,900-kilometre pipeline has been in limbo for more than seven years, and at times has been an irritant in U.S.-Canadian relations.

The project, which also became a focal point of environmental protests, would have shipped bitumen from Alberta's oilsands through a pipeline hub in the Hardisty area to U.S. Gulf Coast refineries.

TransCanada said it had every reason to expect the pipeline would be approved since it met the same criteria as previous pipelines that were sanctioned.

Adam Barratt, a spokesman for Foreign Affairs Minister Stephane Dion, had little to say on the matter Wednesday, although he noted the lawsuit is "not entirely unexpected" and falls within TransCanada's purview.

"We're aware of recent developments with this file and TransCanada," he said. "As this is a matter which is expected to go before arbitration, or before a court, we don't have a comment at this time."