Russia cut off natural gas to Nato members Poland and Bulgaria on Wednesday and threatened to do the same to other countries, using its most essential export in what was seen as a bid to punish and divide the West over its support for Ukraine.

The move, condemned by European leaders as “blackmail,” marked a dramatic escalation in the economic war of sanctions and countersanctions that has unfolded in parallel to the fighting on the battlefield.

The tactic, coming a day after the U.S. and other Western allies vowed to rush more and heavier weapons to Ukraine, could eventually force targeted nations to ration gas and could deal another blow to economies suffering from rising prices. At the same time, it could deprive Russia of badly needed income to fund its war effort.

Poland has been a major gateway for the delivery of weapons to Ukraine and confirmed this week that it is sending the country tanks. Just hours before Russia’s state energy giant Gazprom acted, Poland announced a new set of sanctions against the company and other Russian businesses and oligarchs.

Bulgaria, under a new liberal government that took office last fall, has cut many of its old ties to Moscow and likewise supported punitive measures against the Kremlin. It has also hosted Western fighter jets at a new NATO outpost on Bulgaria’s Black Sea coast

Also, Russian gas deliveries to both Poland and Bulgaria were expected to end later this year anyway.

Still, the cutoff and the Kremlin warning that other countries could be next sent shivers of worry through the 27-nation European Union. Germany, the largest economy on the continent, and Italy are among Europe’s biggest consumers of Russian natural gas, though they, too, have been taking steps to reduce their dependence on Moscow.

“It comes as no surprise that the Kremlin uses fossil fuels to try to blackmail us,” said EU Commission President Ursula von der Leyen. “Today, the Kremlin failed once again in its attempt to sow division amongst member states. The era of Russian fossil fuels in Europe is coming to an end.”

Gazprom said it shut off the two countriesbecause they refused to pay in rubles, as President Vladimir Putin has demanded of “unfriendly” nations. The Kremlin said other countries may be cut off if they don’t agree to the payment arrangement.

Most European countries have publicly balked at Russia’s demand for rubles, but it is not clear how many have actually faced the moment of decision so far. Greece’s next scheduled payment to Gazprom is due on May 25, for example, and the government must decide then whether to comply.

Polish Prime Minister Mateusz Morawiecki told his country’s parliament that he believes Poland’s support for Ukraine — and the new sanctions imposed by Warsaw on Tuesday — were the real reasons behind the gas cutoff.

Bulgarian Prime Minister Kiril Petkov called the suspension blackmail, adding: “We will not succumb to such a racket.”

Ukrainian President Volodymyr Zelenskyy said that Russia views gas as a weapon for political blackmail and “sees a united Europe as a target.”

Russia claimed its missiles hit a batch of weapons that the U.S. and European nations had delivered to Ukraine. One person was killed and at least two were injured when rockets hit a residential neighborhood in Kharkiv.

Western officials, speaking on condition of anonymity to discuss intelligence findings, said Russia has made slow progress in the Donbas region, with “minor gains,” including the capture of villages and small towns south of Izyum and on the outskirts of Rubizhne.

Serhiy Haidai, the governor of the Luhansk region, conceded that Russia has made minor progress in its advance on Rubizhne through its nearly constant bombardment, but that Ukrainian troops are fighting back and retreating only when there is nothing left to defend.

“There is no point of staying at the territory that has been fired so often, that every meter is well known,” he said.

The Western officials said some Russian troops have been shifted from the gutted southern port city of Mariupol to other parts of the Donbas. But some remain in Mariupol to fight Ukrainian forces holed up at the Azovstal steel plant, the last stronghold in the city. About 1,000 civilians were said to be taking shelter there with an estimated 2,000 Ukrainian defenders.