It's now six years since Canadian pipeline operator TransCanada Corp. submitted its initial application to build Keystone XL, a major new crude oil pipeline running from Alberta to Nebraska.

Highlighting the anniversary on Friday, Jack Gerard, president and CEO of the American Petroleum Institute, urged President Obama to finally approve the project.

TransCanada's chief executive, Russ Girling, revealed that the cost of building Keystone XL is expected to almost double by the time the U.S. government concludes its review of the northern leg of the project. The total cost is currently estimated at $5.4 billion, but expenses have increased significantly and could reach $10 billion, the Wall Street Journal reported on Friday.

The proposed pipeline would carry Canadian oil sands to the U.S. Gulf Coast. It has faced controversy and a series of lengthy delays since the U.S. State Department received the initial application in 2008. In 2012 the proposed route was changed after President Obama rejected the plans because of fears about the pipeline's possible environmental impact in Nebraska.

Now, the State Department is awaiting the outcome of a Nebraska Supreme Court case over a state law that approved the pipeline's revised route. That decision may not come until the end of this year or early 2015, the Wall Street Journal said.

The 36-inch-diameter pipeline would transport up to 830,000 barrels of oil a day over a distance of 1,179 miles. It is planned to run from Hardisty, Alta., and extend south to Steele City, Nebraska, giving Canadian and American oil producers better access to the large refining markets in the American Midwest and along the U.S. Gulf Coast.