US Secretary of State Rex Tillerson has recused himself from issues related to a controversial pipeline project, a state department letter said.

The former chief of oil giant ExxonMobil removed himself from discussions concerning the TransCanada Corporation's application for a permit for the Keystone XL pipeline.

"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," a letter from Katherine McManus, the state department’s deputy legal adviser, read on Thursday.

McManus wrote Tillerson had recused himself from the matter in early February.

The letter came after Greenpeace wrote to officials on Wednesday, urging Tillerson to recuse himself on decisions regarding the multibillion-dollar pipeline project because ExxonMobil could benefit from its construction.

"Secretary Tillerson's recent employer, ExxonMobil, is heavily invested in producing crude oil from Canada's tar sands and would directly and predictably benefit from the approval of TransCanada's Presidential Permit," Greenpeace wrote.

BREAKING: State Department announces Tillerson recused himself from #KXL. This is YOUR victory! https://t.co/xQ9s6EOzWC pic.twitter.com/Yc8PyAMPsg — Greenpeace USA (@greenpeaceusa) March 10, 2017

TransCanada had tried for more than five years to build the 1,897km pipeline until former President Barack Obama rejected it in November 2015 amid pressure from environmentalists.

TransCanada resubmitted its application after President Donald Trump signed an executive order in January that helped smoothed the path for its construction.

The project is designed to link existing pipeline networks in Canada and the US to bring crude oil from Alberta and North Dakota to refineries in Illinois en route to the Gulf of Mexico.

Exxon has a majority stake in Imperial Oil, a company that operates an oil sands project in northern Alberta.

During Tillerson's confirmation hearings in January, some senators faulted him for failing to promise to recuse himself from matters related to ExxonMobil businesses for his entire term as secretary of state rather than only the one year required by law.