EU customers' gas supply cannot be guaranteed, according to Russia's Gazprom.

The announcement coincides with a 10-day maintenance period for Nord Stream 1, which transports Russian gas to Germany and other countries.

According to a letter seen by Reuters, Russia's Gazprom has informed clients in Europe that it cannot guarantee gas supplies due to "exceptional" conditions, upping the ante in an economic tit-for-tat with the West over Moscow's invasion of Ukraine.

In a letter dated J....