Russian-owned gas giant, Gazprom, said earlier in the day that it had shut off gas to the two EU nations in retaliation for unpaid energy bills for the month of April when they refused to abide by Moscow’s demands and pay in the Russian currency.
Russian President Vladimir Putin announced the stipulation that all nations must pay gas fees in rubles in late March as an attempt to bolster his flagging economy amid stiff international sanctions.
"The announcement by Gazprom that it is unilaterally stopping delivery of gas to customers in Europe is yet another attempt by Russia to use gas as an instrument of blackmail," EU President Ursula von der Leyen said in a statement.
"This is unjustified and unacceptable. And it shows once again the unreliability of Russia as a gas supplier," she added.
Von der Leyen said that EU nations were "prepared for this scenario" and remained in close contact to identify other means of supplying Poland and Bulgaria with their energy needs.
Poland’s Prime Minister Mateusz Morawiecki further condemned the move by Russia as "blackmail" and vowed in front of his nation's parliament that Warsaw would not be intimidated by Moscow.
Morawiecki said he believed the abrupt cutoff was in retaliation for sanctions Warsaw implemented on 50 Russian oligarchs and businesses, including Gazprom, Tuesday.
Poland received 45 percent of its gas needs from Russia, but the Polish prime minister said his nation would be fine given previous gas arrangements made with other countries.
A new pipeline dubbed the "Baltic Pipe" will direct gas from Norway and Poland – which only relied on nine percent of its energy needs from gas to begin with.
The pipeline is set to be complete by the end of the year.
Warsaw pledged earlier this year to cut gas reliance on Russia by the end of 2022 in retaliation for its deadly war in Ukraine.
The EU said it was working on a "coordinated" response to Russia’s move and vowed to stand behind Poland and Bulgaria.
The Associated Press contributed to this report.