Ukraine has accused Moscow of waging a “gas war” against Europe and cutting supplies to inflict “terror” on people.
Russian energy firm Gazprom announced it is reducing gas flows into Germany to allow work on a turbine on the Nord Stream 1 pipeline.
But Ukraine’s President Volodymyr Zelensky says this is simply “gas blackmail” against Europe.
It comes as hopes remain that grain exports from Ukrainian ports could resume this week following a deal.
Gazprom says on Monday it needed to cut gas supply to around half of current levels in order to carry out maintenance work – but the German government says there is no technical reason for it to limit the supply.
The Nord Stream 1 pipeline, which runs under the Baltic Sea to pump gas from Russia to Germany, has already been running well below capacity for weeks, and was completely shut down for a 10-day maintenance break earlier this month.
Russia supplied the EU with 40% of its gas last year, and the EU has accused Russia of using energy as a weapon.
Zelensky says it is deliberately intended to make it difficult for Europe to prepare for winter, without any care for the poor people may suffer in the colder months as a result.