The United States indefinitely extended the review process for a controversial Canada-to-US oil pipeline Friday, potentially delaying a final decision on the project until beyond mid-term elections in November.

The US State Department said eight federal agencies -- which had been given until the end of May to submit views on the matter -- would now have a longer window to weigh in on the long-delayed Keystone XL pipeline.

The 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 farther south to Texas.

Canadian Prime Minister Stephen Harper expressed disappointment that "politics continue to delay" a decision on Keystone, his spokesman Jason MacDonald said.

"This project will create tens of thousands of jobs on both sides of the border, will enhance the energy security of North America, has strong public support and the US State Department has, on multiple occasions, acknowledged it will be environmentally sound," the statement added.

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 fulfill the US goal of energy self-sufficiency.

Analysts say President Barack Obama, who has the final say on the pipeline, is caught in a classic no-win dilemma -- facing the prospect of losing votes in critical November 4 mid-term elections whatever he decides.

The State Department said that an ongoing legal wrangle in Nebraska, where a judge ruled in February that the proposed route of the 1,179-mile (1,900-kilometer) pipeline was unconstitutional, was partly responsible for the delay.

- 'No intent to delay' -

"Agencies need additional time based on the uncertainty created by the ongoing litigation in the Nebraska Supreme Court, which could ultimately affect the pipeline route in that state," the State Department said.

A State Department official later refused to put a timeline on a final decision.

"We want this to move as expeditiously as possible," the official said.

"I will just underscore that there is no intent to delay the process."

The State Department revealed it had received an "unprecedented" number of comments from members of the public on the pipeline during a consultation period that closed in early March.

"We will review and appropriately consider the unprecedented number of new public comments, approximately 2.5 million, received during the public comment period," it said.

A final decision on a permit would only be taken "once factors that have a significant impact on determining the national interest of the proposed project have been evaluated and appropriately reflected in the decision documents."

The project has caused strains in relations between Ottawa and Washington.

The United States has to approve some 875 miles of the new route. A State Department review of the project released in January concluded the pipeline would have little impact on climate change or the environment.

But the final environmental impact assessment stopped short of making a recommendation on the project. Obama has said previously the pipeline would not be approved if it was likely to increase carbon pollution.

His Republican opponents, staunch supporters of the project, attacked the White House for the latest delay, with House speaker John Boehner decrying it as "shameful."

"This job-creating project has cleared every environmental hurdle and overwhelmingly passed the test of public opinion, yet it's been blocked for more than 2,000 days," Boehner said.

"There are no credible reasons to block this pipeline even one day more."

Democrats from states affected by the project also voiced frustration, accusing Obama of causing an unnecessary delay.

"The administration is sending a signal that the small minority who oppose the pipeline can tie up the process in court forever," said Louisiana Senator Mary Landrieu.

Senator Heidi Heitkamp of North Dakota also slammed the "absolutely ridiculous" delay.

"It hurts all of us when no decisions are made," she added.