Former oil company chief Rex Tillerson will play no part in overseeing an application to build the hugely-controversial Keystone XL pipeline, the US State Department has revealed, amid mounting pressure from environmentalists over his involvement.

The US Secretary of State, who served as chairman and chief executive officer of the energy giant ExxonMobil before accepting a job in Donald Trump’s administration, will recuse himself from an application by a Canadian energy company for a permit to build the pipeline.

The move comes after fears about a conflict of interest were raised.

​TransCanada has filed its application for a US Presidential Permit to build KXL, a proposed crude oil pipeline that, if built, will stretch from Alberta in Canada to Nebraska in the United States.

The State Department said in a letter to Greenpeace Mr Tillerson had removed himself from the project in February.

"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," the letter from Katherine McManus, the State Department's deputy legal adviser, said.

Keystone XL pipeline explained

The recusal came after Greenpeace wrote to officials at the State Department and the Office of Government Ethics, urging Mr Tillerson to distance himself from any decisions on the multibillion-dollar pipeline.

Greenpeace argued in its letter that ExxonMobil would "directly and predictably" benefit from the approval of Keystone XL because the firm has investments in Canadian oil sands.

Barack Obama rejected TransCanada’s application to build the 1,179-mile pipeline in 2015, after five years of attempts to secure the contract.

Trump signs executive orders to push through Dakota Access and Keystone XL pipelines

But in January, TransCanada resubmitted its application after Mr Trump signed an order smoothing its path.

ExxonMobil has a majority stake in Imperial Oil, a Canadian company that operates the Kearl oil sands project in northern Alberta.