WARSAW/SOFIA/KYIV (Reuters) – Russia halted gas supplies to Bulgaria and Poland on Wednesday for rejecting its demand for payment in roubles, taking direct aim at European economies in its toughest retaliation so far against international sanctions over the war in Ukraine.

The step was denounced by European leaders as “blackmail”, and comes as European countries have joined the United States in ramping up arms shipments to help Ukraine fend off a new Russian assault in the east.

Ukraine reported on Wednesday that Russian troops had made gains in several villages there. Russia reported a number of blasts on its side of the border, including a blaze at an arms depot. Kyiv called those explosions “karma”.

Moscow says the gas cut-off is to enforce its demand for payment in roubles, needed to shield its economy from sanctions. Kremlin spokesperson Dmitry Peskov said Russia was a reliable energy supplier and denied it was engaging in blackmail.

Gazprom, the Russian gas export monopoly, said in a statement it had “completely suspended gas supplies to Bulgargaz and PGNiG due to absence of payments in roubles,” referring to the Polish and Bulgarian gas companies.

Polish President Andrzej Duda said the move violated “basic legal principles”.

“Appropriate legal steps will be taken and there will be appropriate compensation from Gazprom for violations of the provisions of the contract.”

Bulgarian Energy Minister Alexander Nikolov said: “It is clear that at the moment the natural gas is being used more as a political and economic weapon in the current war.”

Russian President Vladimir Putin issued a decree last month ordering Gazprom to cut off buyers from “unfriendly” countries that fail to pay in roubles. The European Union says contracts call for payment in euros.

“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,” European Commission President Ursula von der Leyen said.

“This is unjustified and unacceptable.”

Poland and Bulgaria are both former Soviet-era satellites of Moscow that have since joined the EU and NATO. Poland has been one of the Kremlin’s most vocal opponents over the war.

Bulgaria has long had warmer relations with Russia. Prime Minister Kirill Petkov, an anti-corruption campaigner who took office last year, strongly denounced the invasion of Ukraine, but his governing coalition includes lawmakers more sympathetic to Moscow who have opposed arming Kyiv. Petkov was due to travel on Wednesday to Kyiv to meet President Volodymyr Zelenskiy.

Despite the collapse in ties between Russia and the West, Moscow and Washington carried out a prisoner swap. The United States freed Russian pilot Konstantin Yaroshenko, jailed on drug trafficking charges. Russia released former U.S. marine Trevor Reed, accused of fighting with police.

Russia’s energy exports had until now continued largely unhindered since the war began, the main loophole in sanctions that otherwise cut off Moscow from much of its trade with the West.

Poland receives its Russian gas through the Yamal-Europe pipeline from Russia’s huge gas fields in the Arctic far north, which continues west to supply Germany and other European countries. Bulgaria is supplied through pipes over Turkey.

Supplies from Gazprom cover about half of Poland’s consumption and 90% of Bulgaria’s. Poland said it did not need to draw on reserves and its gas storage was 76% full. Bulgaria has said it is in talks to try to import liquefied natural gas through Turkey and Greece.

Other European countries did not report cuts. Peskov, the Kremlin spokesperson, declined to say how many countries had agreed to pay in roubles.

Kyiv has long called on Europe to stop funding Moscow’s war effort by cutting off imports that bring Russia hundreds of millions of dollars a day.

Germany, the biggest buyer of Russian energy, said this week it is hoping to stop importing Russian oil within days. But weaning Europe off cheap and abundant Russian natural gas, which heats its homes, fuels its factories and generates its electricity, would be a far more disruptive prospect.

Andriy Yermak, Zelenskiy’s chief of staff, said Russia was “beginning the gas blackmail of Europe”.

Since the Russian invasion force was driven back at the outskirts of Kyiv last month, Moscow has refocused its operation on eastern Ukraine, starting a new offensive from several directions to fully capture two provinces known as the Donbas.

Ukraine’s general staff acknowledged Russia had made gains in the east, capturing the town of Zavody and some outskirts of the town of Velyka Komyshuvakha on one front, and the Zarichne and Novoshtokivske settlements in Donetsk region.

Blasts were heard early on Wednesday in three Russian provinces bordering Ukraine, authorities said, and an ammunition depot in the Belgorod province caught fire.

Russia this month accused Ukraine of attacking a fuel depot in Belgorod with helicopters and opening fire on several villages in the province. A fire also broke out this week at a fuel depot in nearby Bryansk.

Kyiv has not confirmed responsibility for such incidents but described them as payback. “Karma is a cruel thing,” Ukrainian presidential advisor Mikhaylo Podolyak wrote on social media.

An aide to the mayor of the ruined port city of Mariupol said Russian forces had renewed their attacks on the Azovstal steel plant, where fighters and some civilians are holed up. No agreements had been reached on trying to evacuate civilians from Mariupol on Wednesday.

Concern has increased in recent days over the prospect of the conflict widening to neighbouring Moldova, where pro-Russian separatists have blamed Ukraine for reported attacks this week in a region occupied since the 1990s by Russian troops.

The separatists said firing had taken place across the border from Ukraine near an arms depot on Wednesday.

Kyiv calls the accusations a provocation and Moldova’s pro-Western government blames forces in the separatist region for trying to stir conflict.

The invasion of Ukraine has left thousands dead or injured, reduced towns and cities to rubble, and forced more than 5 million people to flee abroad.

Moscow it a “special operation” to disarm Ukraine and defeat fascists. Ukraine and the West say the fascist allegation is baseless and that the war is an unprovoked act of aggression.