Ursula von der Leyen called Gazprom's decision "yet another attempt by Russia to use gas as an instrument of blackmail" while Charles Michel said it was an "aggressive unilateral move by Russia'.
European leaders on Wednesday branded Russia's announcement it is cutting off gas supplies to Poland and Bulgaria as "blackmail" and praised EU solidarity as neighbouring countries step in.
Gazprom, Russia's state-owned gas company, informed both EU countries on Tuesday that it would halt gas supplies after they refused to pay for the deliveries in roubles — a measure Moscow imposed on so-called "unfriendly" foreign buyers in response to sanctions over its invasion of Ukraine.
About half of Poland's imported gas comes from Russia, according to Forum Energii, a think tank. The share rises even further for Bulgaria which gets at least three-quarters of its gas imports from Russia.
Commission chief Ursula von der Leyen labeled Gazprom's decision as "yet another attempt by Russia to use gas as an instrument of blackmail" in a statement on Wednesday morning.
She added that it is "unjustified and unacceptable" but sought to strike a reassuring note, stressing that the 27-country bloc is "prepared for this scenario" and has "put in place contingency plans for just such a scenario and we worked with them in coordination and solidarity."
European Council President Charles Michel, for his part, branded the move "another aggressive unilateral; move by Russia."
He added that he is in contact with Mateusz Morawiecki and Kiril Petkov, respectively Polish and Bulgarian prime ministers.
Petkov said meanwhile that he has been in touch with Kyriakos Mitsotakis, leader of neighbouring Greece to discuss the situation with both vowing to "continue to work together for energy security and diversification."
He also added that they are "confident" that the Gas Interconnector Greece-Bulgaria (IGB) will be completed "on time". The pipeline will allow Bulgaria to be connected to the Southern Caspian Corridor which travels through Turkey, Georgia and Azerbaijan to both Greek and Italian terminals.
The gas cuts do not immediately put the countries into dire trouble since they have worked on getting alternative sources for several years now and the continent is heading into summer, making gas not as essential for households.
Still, it sent shivers of worry through the 27-nation European Union, which immediately convened a special coordination group to limit the impact of the move.
Von der Leyen told reporters that the EU's response to this "provocation from the Kremlin" will be "immediate, united and coordinated".
She said Warsaw and Sofia have updated the rest of the bloc on the situation and are now "receiving gas from their EU neighbours", which she stressed showed the "immense solidarity among us" as well as the "effectiveness of past investments" in infrastructure such as interconnectors.
She reiterated that the Commission reached an agreement earlier this year with the US to provide additional energy imports this year and in the coming years and that Brussels is also working to secure alternative gas supplies from other partners.
Gazprom's decision, she added, is "a stark reminder that we need to work with reliable partners and build our energy independence."
"Today, the Kremlin failed once again in its attempt to sow division among member states," she claimed.
Asked about a Bloomberg report which stated that several European companies have since agreed deals to pay in roubles to avoid being cut off, the Commission chief highlighted that paying in the Russian currency would violate EU sanctions and that 97% of the bloc's gas contracts with Russia were signed in euros or dollars.
Kremlin spokesman Dmitry Peskov told reporters earlier in the day that other European customers may be cut off if they also refuse to pay in roubles.