The International Court of Justice (ICJ) on Wednesday declared that states were obliged under international law to tackle climate change, and failing to do so could leave them open to being sued.

The ICJ said climate change was an "urgent and existential threat" and countries had a legal duty to prevent harm from their planet-warming pollution.

Countries breaching their climate obligations were committing a "wrongful act", the court said in its advisory opinion, which is not legally binding but carries political and legal weight.

Campaigners and countries on the climate frontlines hailed the ruling as an important moment in the fight for accountability from big polluters most responsible for global warming.

The German foreign ministry on Thursday said the ICJ's opinion confirmed that "climate protection is the duty of all states". It described the ruling as an "important milestone".

The EU said the "important" decision "only confirms the immensity of the challenge we face and the importance of climate action and the Paris Agreement".

"It also reaffirms the need of taking collective and ambitious action," Anna-Kaisa Itkonen, a spokeswoman for the European Commission, told journalists.

China also welcomed the "positive" ruling.

"The advisory opinion reflects the long-term positions and propositions of the vast majority of developing countries, including China, and has positive significance for maintaining and promoting international climate cooperation," said foreign ministry spokesman Guo Jiakun.

The British foreign ministry said it would take time to examine the advisory opinion before commenting in detail.

"Tackling climate change is and will remain an urgent UK and global priority," the statement said.

"Our position remains that this is best achieved through international commitment to the UN's existing climate treaties and mechanisms."

The United States, which has embraced a fossil-fuel agenda under President Donald Trump, had on Wednesday given a muted response to the ruling.

A US State Department spokesperson said it "will be reviewing the Court's advisory opinion in the coming days and weeks".

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