The US state department has issued a presidential permit that gives energy firm TransCanada the green light to build the controversial Keystone XL pipeline.

The move on Friday reversed a decision by ex-President Barack Obama who had blocked the project over environmental worries.

The state department said the $8bn Keystone project would serve US national interest, opposite to the conclusion it had made two years ago.

Announcing the permit, President Donald Trump called the Keystone project an "incredible pipeline".

"TransCanada will finally be allowed to complete this long overdue project with efficiency and with speed," Trump said.

"It's a great day for American jobs and an historic moment for North America and energy independence," he added. 

READ MORE: Trump clears Keystone XL, Dakota Access pipelines

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

Yet, Trump invited the company to resubmit it application in January, and signed an executive order that helped smooth the path for its construction.

TransCanada called Friday's decision a "significant milestone".

"We greatly appreciate President Trump's administration for reviewing and approving this important initiative," Russ Girling, the company's chief executive, said.

"We look forward to working with them as we continue to invest in and strengthen North America's energy infrastructure."

The company also said it would drop its lawsuit that called Obama's 2015 decision unconstitutional.

Al Jazeera's Shihab Rattansi, reporting from Washington, DC, said a number of environmental groups responded to Friday's decision by saying that they were mobilising and ready to continue fighting against the pipeline.

"It isn't over yet, the pipeline still needs a permit in the states it will traverse," Rattansi said.

READ MORE: Tillerson recuses himself from Keystone pipeline issues

The permit was signed by Tom Shannon, a career diplomat serving as undersecretary of state for political affairs, because Secretary of State Rex Tillerson recused himself due to his work as the former chief of oil giant ExxonMobil.

Portions of Keystone have already been built, but its completion required a permit to cross from Canada into the US.

The project is designed to link existing pipeline networks in the two countries to bring crude oil from Alberta and North Dakota to refineries in Illinois en route to the Gulf of Mexico. The pipeline would move roughly 800,000 barrels of oil per day, more than one-fifth of the oil Canada exports to the US.

Oil industry advocates have argued the pipeline will improve US energy security and create jobs, but the project’s opponents have said the job creation will be minimal and short-lived.

'Government moving backwards'

Environmentalists have long fought against the project, arguing it would encourage the use of carbon-heavy tar sands oil which contributes more to global warming than cleaner sources of energy.

Canada-US pipeline project meets resistance

In a statement on Friday, Greenpeace said the permit sends a signal to the world that the "government is moving backwards when it comes to climate and energy".

The environmental group also vowed to fight to stop TransCanada's funding sources for the project.

"It takes money to build a pipeline, and the opposition movement to stop fossil fuel projects like Keystone will do everything it can to deprive TransCanada of any new funding for this ill-fated and unnecessary pipeline. TransCanada may have a permit, but can they find the funding?," Greenpeace said.

Trump has championed the pipeline and backed the idea that it will create jobs. Under January's executive order, new or expanded pipelines must be built with US steel "to the maximum extent possible".

TransCanada, however, has said Keystone will not be built with US steel as much the required steel has already been purchased from Canada and Mexico. The White House has acknowledged it is too difficult to impose conditions on a pipeline already under construction.

On Friday, Trump also said the Keystone project is one of a number of new energy and infrastructure projects his administration hopes to see completed. 

"As the Keystone XL project now moves forward this is just the first of many energery and infrastructure projects that my administration will approve and we've already approved a couple of other bigs ones...which we will be announcing soon," Trump said. 

Source: Al Jazeera and news agencies