A dragging review into proposals to build a pipeline from Canada's oil sands to the US Gulf Coast may be further delayed due to the US government shutdown, an official said Friday.

The proposed 1,179-mile (1,897 km) Keystone XL pipeline -- to carry oil extracted from Alberta's tar sands south to American refineries -- has been on hold for years, awaiting a green light.

The State Department is putting together a final environmental impact statement to determine whether to give the $5.3 billion scheme the go-ahead.

The project was proposed back in 2008, but after years of delays, operator TransCanada split it in two, and construction began on a southern section that does not require presidential approval.

But the review on the impact of the northern section, which would cross through environmentally sensitive areas in Nebraska, is ongoing.

"Finalizing the draft involves work with consulting agencies to discuss and address their comments," said State Department deputy spokeswoman Marie Harf.

"Most of these consulting agencies have had a large number of staff furloughed during this process, which has made it harder to work on with them," she told reporters.

"We obviously need information, technical expertise that these agencies can provide, and it's just making it more difficult now."

For several months, the State Department, which is reviewing the scheme because it crosses the border from Canada, has refused to say when the analysis will be ready. President Barack Obama will likely give any final seal of approval.

TransCanada said earlier this month it did not believe it would be given the White House nod before the end of the year.