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The company behind the Keystone XL pipeline has asked the US government to put its review of the controversial project on hold.

TransCanada says the pause is necessary while it negotiates with Nebraska over the pipeline's route through the state.

The move came as a surprise as TransCanada executives have pushed hard to get approval.

Environmental groups oppose the 1,179-mile (1,897km) pipeline, saying it will increase greenhouse gas emissions.

President Barack Obama is expected to reject the project, which has also been undermined by falling oil prices.

On Monday the White House indicated that it would rule on the project before the end of the president's term in office in January 2017.

Political complications

But a delay to the government review might leave a decision in the hands of President Obama's successor in the White House.

In February 2015, the newly Republican-led Congress voted to begin construction immediately, but Mr Obama vetoed the bill, saying it undermined the necessary review process.

"Our expectation at this point is that the president will make a decision before the end of his administration on the Keystone pipeline, but when exactly that will be, I don't know at this point," White House spokesman Josh Earnest told reporters on Monday.

In a statement on Monday TransCanada chief executive Russ Girling said: "We are asking (the US) State (Department) to pause its review of Keystone XL based on the fact that we have applied to the Nebraska Public Service Commission for approval of its preferred route in the state."

The Keystone XL pipeline would run from the oil sands in Alberta, Canada, to Steele City, Nebraska, where it could join an existing pipeline.

It could carry 830,000 barrels of oil each day, and provide access to international markets.

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Many of North America's oil refineries are based in the Gulf Coast, and industry groups on both sides of the border want to benefit.

But environmentalists say the pipeline would boost the emission of greenhouse gases and local community groups are concerned about accidents and pollution.

An added complication is the victory of Canada's Liberal Party in last month's election.

New Prime Minister Justin Trudeau has been supportive of the pipeline, but it is thought he might not pursue the project as aggressively as his predecessor Stephen Harper, who said he wouldn't "take no for an answer" from President Obama.