Environmental defenders rallied behind Greenpeace after the shock ruling by a North Dakota jury fuelled concerns that courtrooms were increasingly being used to smother critics.

"It sends a dangerous message: that fossil fuel giants can weaponize the courts to bankrupt and silence those who challenge the destruction of our planet," said Anne Jellema, executive director of advocacy group 350.org.

The judgement "is not only an attack on Greenpeace -- it is an assault on the entire climate movement, clearly intended to chill the resistance to fossil fuels", she added in a written statement to AFP.

Energy Transfer (ET), the Texas-based pipeline operator awarded the damages, has denied any attempt to stifle free speech by suing Greenpeace.

The company had accused the environmental advocacy group of orchestrating violence and defamation during the construction of the contentious Dakota Access Pipeline project nearly a decade ago.

The jury awarded more than $660 million in damages across three Greenpeace entities, citing charges including trespass, nuisance, conspiracy, and deprivation of property access.

Brice Bohmer from Transparency International, a global corruption watchdog, said the lawsuit was "unconscionable" but evidence of a much wider problem.

"This kind of activity is becoming increasingly common across climate action, with fossil fuel actors undermining progress wherever possible," he said.

- Global threat -

ET initially sought $300 million in damages through a federal lawsuit, which was dismissed.

It then shifted its legal strategy to North Dakota's state courts -- one of the minority of US states without protections against so-called "Strategic Lawsuits Against Public Participation" or SLAPPs.

Throughout the years-long legal fight, ET's billionaire CEO Kelcy Warren, a major donor to President Donald Trump, was open about his motivations, saying in interviews that he wanted to "send a message".

Matilda Flemming, director of Friends of the Earth Europe, said she was "appalled" by the verdict but warned it was not an isolated case.

"The right to protest is under threat across the world, from big corporations and self-interested politicians who threaten our democracies," she said.

Greenpeace has vowed to continue its advocacy and its international body is counter-suing ET in the Netherlands, accusing the company of nuisance lawsuits to stifle dissent.

Rebecca Brown, president and CEO of the Center for International Environmental Law (CIEL), said the fight for environmental justice would go on.

"No abusive company, lawsuit, or court decision will change that," she said in a statement on Wednesday after the verdict was handed down.