Calgary-based TransCanada, in a formal letter to U.S. Secretary of State John Kerry, has requested that the Department put off its review of the Presidential Permit application for the Keystone XL pipeline.

According to the New York Times, the letter has been widely interpreted as an attempt to avert an impending "no" from President Barack Obama to the nearly 1,200-mile (2,000-km) $8 billion cross-border pipeline. Keystone XL would carry heavy crude oil from Alberta to Nebraska and on to Gulf Coast refineries, and has become the center of a struggle between environmentalists opposed to oil sands development and defenders of fossil fuels.

The U.S. State Department said it had received a letter from TransCanada asking for the delay but a spokesperson said the review would continue for now.

Obstacles to the project are growing, not least of which are Obama's personal reservations concerning the environmental impact of a pipeline transporting tar sands crude. Furthermore there has been repeated wrangling between landowners in Nebraska - where the pipeline route cuts across - and the Canadian company. And with newly elected Prime Minister Justin Trudeau's rather muted support up to now, the prospect of success seems slim indeed.

Obama is expected to make a decision on Keystone before he leaves office in January 2017.