Delays in greenlighting TransCanada's Keystone XL pipeline will increase construction costs and postpone its in-service date by at least six months, the company said Friday.

"Due to ongoing delays in the issuance of a (US) presidential permit for Keystone XL, we now expect the pipeline to be in service in the second half of 2015," TransCanada said in its quarterly financials.

"Based on our pipeline construction experience, the $5.3 billion cost estimate will increase depending on the timing of the permit."

TransCanada originally hoped to have finished by early 2015 building the 1,179-mile (1,897-kilometer) pipeline from the Canadian province of Alberta to the US state of Nebraska, where it would hook up with a new southern leg to bring the oil to refineries in Texas.

But the proposal stirred up a lot of controversy.

Pipeline supporters say the project would generate much-needed jobs for the sluggish US economy, while opponents warn it could have a dire impact on the environment and vital groundwater resources.

The State Department is expected to make a final recommendation on the project to US President Barack Obama in the coming months.