Michel Forst, the UN special rapporteur on environmental defenders, described as "highly troubling" the Norwegian supreme court rulings in December, which handed the protesters prison time and steep fines.

Forst is an independent expert appointed to monitor compliance with the UN's Aarhus Convention, which provides for justice in environmental matters.

He warned in a statement that imposing harsh sentences for the offence of "aggravated damage" violated the convention.

The two judgements "may demonstrate a lack of understanding of Norway's obligations under ... the Aarhus Convention in relation to the protection of persons engaging in peaceful environmental protest", he said.

Two of the activists, Anne Klenge and Joachim Skahjem, had sought in November 2022 to highlight Norway's failures in reducing carbon emissions and its ongoing oil exploration policy with an action in Oslo's Vigeland Sculpture Park.

Forst said they had thrown "washable water-based paint" on the Monolith, which towers over the park, as well as several other nearby sculptures.

A year later, Esther Hjerrild and Fridtjof Klareng Dale "sprayed washable water-based paint" on the exterior of Norway's climate and environment ministry.

All four activists were ultimately sentenced to prison terms of up to 50 days, while Hjerrild and Klareng Dale received a particularly steep compensation claim of nearly 1.2 million kroner ($120,000), which they were ordered to pay within two weeks of the verdict.

"In a highly troubling development, the supreme court decided to impose even harsher sentences on the environmental defenders than the lower courts had done," Forst said.

The expert, who does not speak on behalf of the UN, stressed that Norway as a party to the Aarhus Convention had a "specific obligation to ensure the protection of environmental defenders".

And "while law-breaking during a peaceful environmental protest may be sanctioned", he added, it must be "reasonable, proportional and pursue a legitimate public purpose".

Forst said the sanctions instead appeared to be "punitive and repressive", warning that the "criminalisation of persons engaged in peaceful environmental protest and civil disobedience is a major threat to democracy".

"This must sound the alarm for any member of the public in Norway who cares about the environment and maintaining a safe civic space," he said.