OSLO- Greenpeace and Extinction Rebellion campaigners blocked a tanker from delivering Russian oil to Norway on Monday, chaining one of their boats to the giant vessel in a protest against the war in Ukraine, Greenpeace said.

The Ust Luga product tanker is anchored outside Exxon Mobil's Slagen oil terminal about 70 km (43 miles) south of the capital Oslo, according to vessel tracker Marine Traffic.

Activists in a red rubber dinghy chained their vessel to the tanker's anchor chain as they sought to prevent the offloading of a cargo estimated at 95,000 tonnes of oil, Greenpeace said in a statement.

Several demonstrators were arrested, local police said.

"Oil is not only at the root of the climate crisis, but also of wars and conflicts. I am shocked that Norway operates as a free port for Russian oil, which we know finances Putin's warfare," Greenpeace Norway head Frode Pleym said.

The group called on the Norwegian government to ban imports of Russian fossil fuels and said that Exxon Mobil's Norwegian Esso unit should cancel any contracts for such imports from Russia over the invasion of Ukraine.

Esso Norway had agreed to buy the oil before the conflict started and does not have plans for further purchases from Russia, a company spokesperson told Reuters.

"Esso Norway fully complies with all Norwegian sanctions and we support the coordinated international efforts to end Russia's unprovoked attack," Esso said in an emailed statement.

The tanker's cargo is used for the blending of marine gasoil for the shipping industry, the company added.

Russia has described its aims in Ukraine as a "special military operation" to disarm its neighbour and defeat nationalists there. Ukraine and the West reject that as a baseless pretext for an unprovoked war of aggression.

(Reporting by Terje Solsvik ;editing by Louise Heavens and David Goodman )