US Secretary of State Rex Tillerson has recused himself from the decision over the Keystone Pipeline, the State Department has said.

TransCanada, a Canada based Energy Company, submitted an application last month to move ahead with the pipeline to bring crude oil from Alberta's tar sands to US refineries.

On Wednesday, environmental group Greenpeace questioned Tillerson's links to ExxonMobil, the oil giant where he was CEO before taking the helm at the State Department, which it says would benefit from the pipeline's approval.

Acting State Department Spokesperson, Mark Toner said: "Secretary Tillerson decided in early February to recuse himself from TransCanada's application for a Presidential Permit for the proposed 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."

The pipeline had been rejected by former president Barack Obama in 2015. However, in one of his first acts as president, Donald Trump signed an executive order advancing construction of the Keystone and Dakota Access pipelines. Trump had directed the Secretary of State to come to "a final permitting determination" within 60 days of a renewed TransCanada application.

Greenpeace responded to the announcement by calling for more government transparency: "Rex Tillerson's recusal from the Keystone Pipeline decision 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."

"Consider adding more phone lines"

Senior Climate and Energy Campaigner for Greenpeace, Diana Best, said "the people will have to watch Trump's cabinet of billionaires and corporate executives like hawks" before adding in a vague threat about how hard the group plans to push the current administration: "The Office of Government Ethics should consider adding more phone lines."

Tillerson's move may seem a small victory for environmental activists on the day that Scott Pruitt, head of the Environmental Protection Agency, said that he did not think carbon dioxide emissions are "a primary contributor to the global warming that we see".

There has also been speculation that the current administration might pull out of the Paris Climate Change agreement, signed in 2016 by 194 countries in an attempt at helping global cooperation on fighting the threats posed by climate change.

A senior State Department official said this week: "We're looking at the agreement, [Secretary Tillerson] has recognised that climate change is an issue but, like many of our policies and many of our agreements, all this is still under review."

Asked about the Paris agreement during his confirmation hearing, Tillerson said that he thought it was important that "the US maintains its seat at the table about how to address the threat of climate change".