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.
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 would enter into force the following morning.