WASHINGTON • United States Secretary of State Rex Tillerson, a former oil executive, has recused himself from any role in approving the Keystone XL pipeline between Canada's tar sands and US refineries.

President Donald Trump has signalled that he will green-light the project, which his predecessor, Mr Barack Obama, had put on hold after a report overseen by the State Department endorsed environmental concerns.

After Mr Trump promised during his successful presidential campaign that he would approve the 1,900km pipeline, Mr Tillerson's department is due to produce a new report that is expected to be positive.

But environmental activists, including Greenpeace, have said that Mr Tillerson - the retired chief executive of America's biggest energy giant ExxonMobil - has a conflict of interest and should not have a hand in the new recommendations.

"We can confirm that the department received the letter from Greenpeace on March 8," said State Department spokesman Mark Toner.

"Secretary Tillerson decided in early February to recuse himself from TransCanada's application for a presidential permit for the proposed Keystone XL pipeline," he added. "He has not worked on that matter at the Department of State, and will play no role in the deliberations or ultimate resolution of TransCanada's application."

Calgary-based TransCanada announced in late January after Mr Trump's inauguration that it would resubmit its application to the State Department to build a pipeline from Canada's oil sands to the US Gulf Coast. Environmentalists say tar sand oil contains a corrosive component - bitumen - which makes pipeline ruptures or leaks more likely and carries greater health and safety risks. But TransCanada says that buried pipelines of the kind it plans for Keystone XL are far safer for transporting oil than ships or trains.

It also argues that bringing another 830,000 barrels a day from neighbouring Canada would reduce US dependence on the unstable Middle East and Venezuela by up to 40 per cent.

AGENCE FRANCE-PRESSE