A Polish diplomat told EUobserver on Thursday (11 September) that volumes fell by 20 percent on Monday and were down by 45 percent by Thursday.
The same day Slovak PM Robert Fico said supplies to his country had dipped by eight to 11 percent.
Both countries are trying to find out what is going on amid denials by Russian supplier Gazprom that its shipments are any lower than normal.
The Polish diplomat said the enquiries are being made by Polish gas distribution firms. But Fico told press he has tasked officials to contact Moscow.
Poland, Slovakia, and Hungary are involved in shipping “reverse-flow” gas to Ukraine after Russia cut off Ukraine gas in a price dispute.
But a Hungarian diplomat told this website its Russia gas deliveries are normal for now.
“We did not see a drop in supplies. But we are filling up our storage tanks so that we are prepared for any scenario that may come”, the source noted.
The Polish diplomat added: “On the Polish side, there is no risk to national consumers at this stage. We have generous reserves. But the supply reductions could be a burden on reverse-flow and I have heard talk that this is being scaled down”.
Gazprom’s reductions come the same week the EU adopted a new round of economic sanctions against Russia following its invasion of east Ukraine.
Some EU states had wanted to wait and see if a ceasefire deal with Russia holds before implementing the new measures.
But EU Council chief Herman Van Rompuy announced on Thursday that they will enter into life the following morning.
The most vulnerable to gas cuts is Finland and according to report by EUobserver
“A Russian gas export embargo during the winter of 2014/15 lasting for more than 6 months would cause supply shortfalls in many European countries, in particular, in central and eastern Europe, including Germany,” according to a study published Wednesday (3 September) by the Institute of Energy Economics at the University of Cologne.