Moscow and Kiev are locked in a stand-off over the price of Russian gas, with Russia saying it may reduce supplies if Ukraine fails to pay an increased price and pay off its debts.
The source, who declined to be named, said the latest available data for Russian gas exports to Ukraine showed deliveries on Wednesday of 48.9 million cubic metres (mcm), down from around 80 mcm on April 1 and 65 mcm on Monday and Tuesday.
Russian natural gas producer Gazprom declined to comment.
The volume of gas exports to Ukraine fluctuated earlier this year and analysts say the weather is getting warmer, which may account for reduced demand. Last year, Ukraine imported 25.8 billion cubic metres of gas from Russia, or over 70 mcm a day on average.