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The US secretary of state has stepped away from dealing with issues related to the controversial Keystone pipeline, because he used to run an oil company.

Rex Tillerson, former boss of Exxon Mobil, recused himself from the matter in February, the State Department said.

TransCanada Corp has applied for a permit for the Keystone XL pipeline, the State Department said.

It resubmitted its application in January after President Donald Trump restarted the project.

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"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," said a State Department letter sent to the environmental group Greenpeace on Thursday.

The State Department declined to say what prompted Mr Tillerson's decision, but Greenpeace had requested he recuse himself because his previous employer would benefit from the pipeline's construction.

Ethical concerns persist - Anthony Zurcher, BBC News, Washington

Donald Trump's Cabinet of high-flying business executives and billionaires with far-flung financial interests continues to generate challenges for his new administration. Ethical concerns have repeatedly presented themselves during the Senate confirmation process - and those challenges are obviously not going away anytime soon.

The fact that Rex Tillerson made this recusal decision without informing the public is also curious. His relation with the media has been strained at best - mostly non-existent, in fact. He'll be heading to Asia next week without bringing any reporters along with him, representing a break with longstanding tradition.

Again we are reminded that this is a different kind of administration, full of many non-politicians who have a decidedly different way of handling the media.

Mr Tillerson was sworn in as secretary just a week after the president signed an executive order to move forward with the controversial pipeline's construction.

The project entails a planned 1,179-mile (1,897km) pipeline running from the oil sands of Alberta, Canada, to Steele City, Nebraska, where it would join an existing pipe. It could carry 830,000 barrels of oil each day.

Media playback is unsupported on your device Media caption Rajini Vaidyanathan: "Separating policy from profit could be challenging"

In response to the State Department, Greenpeace issued a new statement saying his recusal "might have never been transparent to the public without people flooding the lines of the Office of Government Ethics today".

"We must keep pushing this administration into the spotlight and demanding that ethics watchdogs hold these individuals accountable and make these decisions regarding rampant conflicts of interest transparent," it continued.

It remains unclear whether Mr Tillerson still owns stock in ExxonMobil.

But the secretary of state has said he would fully divest from the company by May to comply with federal ethics rules.

In State Department briefings earlier this week, acting spokesman Mark Toner said the secretary was "working with the Office of Government Ethics" on the issue.