The European Union warned Russia on Wednesday it would not bend to "blackmail" over its support for Kyiv, after the Kremlin cut off gas supplies to Bulgaria and Poland.
Russia's President Vladimir Putin, meanwhile, warned that if Western forces intervene in Ukraine, they will face a "lightning-fast" military response.
"We have all the tools for this, that no one else can boast of having," he told lawmakers, implicitly referring to Moscow's ballistic missiles and nuclear arsenal.
"We won't boast about it: we'll use them, if needed. And I want everyone to know that," he said. "We have already taken all the decisions on this."
The dire threats came as Moscow claimed to have carried out a missile strike in southern Ukraine to destroy a "large batch" of Western-supplied weapons.
As the war, which has already claimed thousands of lives, entered its third month, Kyiv conceded that Russian forces had made gains in the east.
Russia's military offensive saw it capture a string of villages in the Donbas region, now the immediate target of its invasion force.
And in its economic standoff with the West, Moscow cut off gas supplies to Bulgaria and Poland, two EU and NATO members backing Ukraine in the conflict.
In Brussels, Ursula von der Leyen, the president of the European Commission, insisted that EU member states were ready for the move.
'Blackmail'
She described the announcement by Russia's state energy giant Gazprom as "another provocation from the Kremlin" that would be countered.
"It comes as no surprise that the Kremlin uses fossil fuels to try to blackmail us... Our response will be immediate, united and coordinated.
"Both Poland and Bulgaria are now receiving gas from their EU neighbors," she said. "The era of Russian fossil fuels in Europe will come to an end."
European powers have imposed massive sanctions on Russia since Putin's decision to invade his neighbor, while shipping weapons to Ukraine's defenders.
But they have moved slowly on hitting Moscow's vast gas exports, with many EU members -- notably industrial giant Germany -- reliant on Russian energy to keep their lights on.
Putin has attempted to turn up the pressure by insisting that Russia will only accept payments for gas in rubles -- hoping to force his foes to prop up his currency.
Gazprom announced the halt of gas to both Poland and highly dependent Bulgaria, saying it had not received payment in rubles from the two EU members.
But von der Leyen said that "about 97 percent" of all EU contracts explicitly stipulate payments in euros or dollars -- and warned importing firms off paying in rubles.
"This would be a breach of the sanctions," she told reporters.
'Heaven and earth'
Moscow defended its demand that western customers buy rubles, saying that sanctions against its central bank had forced it to rebuild its foreign reserves.
"They blocked from us -- or, to put it plainly, stole -- a fairly significant amount of our reserves," Kremlin spokesman Dmitry Peskov said. "So there is no question of blackmail here."
The first phase of Russia's invasion failed to reach Kyiv and to overthrow President Volodymyr Zelensky's government after encountering stiff Ukrainian resistance reinforced with Western weapons.
The campaign has refocused on seizing the east and south of the country, while increased use of long-range missile strikes against west and central Ukraine to counter the Western response.
On Wednesday, Russia's defense ministry said its forces had destroyed a "large batch" of weapons and ammunition supplied by the United States and European countries.
Russia hit hangars at an aluminum plant near the Ukrainian city of Zaporizhzhia with "high-precision long-range sea-based Kalibr missiles", the ministry said.
On Tuesday, at a summit in Germany of 40 Western allies to discuss arming Ukraine, Washington pledged to move "heaven and earth" to enable Kyiv to emerge victorious from the war.
Tensions are also rising in a breakaway region of Moldova bordering southwestern Ukraine.
In the region, Transnistria, pro-Russian separatists claimed shots were fired across the border towards a village housing a Russian arms depot, after drones flew over from Ukraine.
'Ramp up'
The unrecognized region has reported a series of explosions in recent days that it called "terrorist attacks", leading Kyiv to accuse Moscow of seeking to expand the war further into Europe.
The targeting of Western-supplied arms came as the United States and Europe have started to heed Zelensky's call for heavier firepower.
Western allies remain wary of being drawn into war with Russia but have stepped up military support as Ukraine has maintained its fierce resistance.
Germany announced Tuesday it would send anti-aircraft tanks, in a sharp U-turn on its much-criticized cautious stance.
Britain will on Wednesday urge Kyiv's allies to "ramp up" military production including tanks and planes to help Ukraine, with Foreign Secretary Liz Truss set to call for a "new approach" to confront Putin.
Fighting continues to rage across Ukraine's east, Kyiv's defense ministry said, as it confirmed Russian forces had seized several villages in their renewed bid to "liberate" the Donbas region.
The ministry said a pair of villages in the north-eastern Kharkiv region and two in the Donetsk region had fallen.
Meanwhile, three people died and 15 others were injured in bombings around the eastern city of Kharkiv, Ukraine's second city, regional governor Oleg Synegubov said.
Moscow aims to create a land bridge between territory held by pro-Russian separatists in parts of the Donbas and the Russian-annexed Black Sea peninsula of Crimea.
Separately, Moscow also said it was expelling eight Japanese diplomats in a tit-for-tat response to expulsions by Tokyo over the conflict in Ukraine.
