Poland's PGNiG gas utility on Wednesday said its gas deliveries from Russia's Gazprom had been cut by 24 percent, amid heightened tensions over the Ukraine crisis.
"A decline in deliveries of approximately 24 percent compared to the volumes ordered was noted" on Tuesday on pipelines transiting from Ukraine and Belarus, PGNiG said in a statement.
"We are trying to determine the reasons behind this, specifically whether they are technical or commercial in nature."
PGNiG added that it had observed a first cut in deliveries on Monday. A spokesman told AFP it was plugging the gap in supplies with deliveries "from the south and the west," without elaborating.
A source at Gazprom meanwhile said the Russian gas giant was "in the process of verifying" the information.
Poland is highly dependent on Russian gas. Of the 16 billion cubic metres of gas it uses annually, over 60 percent is imported, mainly from Russia.
But tensions between the two countries have risen over the escalating conflict in Ukraine between government forces and pro-Kremlin separatists. 
Poland has been one of the staunchest supporters of Kiev's pro-West government and has repeatedly called for tougher sanctions against Moscow.
Russia has halted gas supplies to Ukraine over the crisis, forcing Kiev to look elsewhere for deliveries. 
German group RWE has notably been supplying Ukraine with gas since April, sending the so-called "reverse flow" deliveries through Poland.