Russia's Gazprom has declared force majeure on gas supplies to Europe to at least one major customer, according to the letter dated July 14 and seen by Reuters on Monday.
The letter said Gazprom, which has a monopoly on Russian gas exports by pipeline, could not fulfil its supply obligations owing to “extraordinary” circumstances outside its control.
A trading source said the letter concerned supplies through the Nord Stream 1 pipeline, a major supply route to Germany and beyond.
Gazprom had no immediate comment.
Invoking force majeure, used when a business is hit by something beyond its control, will likely escalate tensions between Russia and the West over the Russian invasion of Ukraine.
The EU, which has imposed sanctions on Moscow, aims to stop using Russian fossil fuels by 2027 but wants supplies to continue for now as it shifts away from Russian supplies.
Russian gas supplies have dropped via major routes, including through Ukraine and Belarus and through Nord Stream 1 under the Baltic Sea.
Nord Stream 1 is currently undergoing maintenance.