US Secretary of State John Kerry Wednesday refused to give away any hint on whether he will approve a controversial Canada to US oil pipeline, saying he was intensely studying the plan.

The Keystone XL pipeline, first proposed in 2008, is slated to cross US borders bringing oil from the tar sands of Alberta to refineries in the US state of Nebraska and then further south to Texas.

But it has been long delayed awaiting an environmental review from the State Department, and a final recommendation from Kerry, who has long been a passionate advocate for the environment.

Asked if last week's ruling by a Nebraska judge that the proposed route of the 1,179-mile (1,897-kilometer) pipeline was unconstitutional would affect his thinking, Kerry refused to be drawn.

"It might or might not have a substantive impact on it," Kerry told a small group of reporters including AFP.

"Really right now, I'm doing my due diligence on it," Kerry said, adding "I want to do it in a complete 'tabula rasa' approach" using the Latin for a "blank slate."

"I just want to look at the arguments, I want to look at all that's been done. I haven't done it yet purposefully. I've stayed away from it, which is what I'm supposed to do," he said.

"And now I'm entering a very intensive evaluation."

The project has pitted environmental groups against the oil industry, which has argued that it will bring much-needed jobs to the United States and help fulfil the US goal of energy self-sufficiency.

It also has caused strains in relations between Ottawa and Washington. The United States has to approve some 875 miles of the new route.

An independent watchdog meanwhile Wednesday released an assessment of how the State Department has handled the environmental review process.

The inspector general found that the selection process used to pick a third-party contractor, Environmental Resources Management (ERM), to write the review did not break any rules on conflict of interest.

Activists Friends of the Earth have alleged that ERM has ties to the pipeline operators TransCanada.

Welcoming the inspector general's findings, State Department spokeswoman Jen Psaki said it "found that our processes not only avoided conflicts of interest, but were more rigorous than required."