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 said this was simply "gas blackmail" against Europe.
It comes as hopes remain that grain exports from Ukrainian ports could resume this week following a deal.
Gazprom said 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 said there was 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.
"The gas blackmail of Europe, which only gets worse every month, is needed by a terrorist state to make the life of every European worse," said Mr Zelensky in his nightly address.
He said it was deliberately intended to make it difficult for Europe to prepare for winter, without any care for the poverty people may suffer in the colder months as a result.