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.
The warning came as UN chief Antonio Guterres arrived in Kyiv to meet Ukrainian leader Volodymyr Zelenskyy following talks with Russian President Vladimir Putin in Moscow to expand humanitarian support and secure civilian evacuations.
Mr Putin issued his own warning on Wednesday, saying that if Western forces intervene in Ukraine, they will face a "lightning-fast" military response, after telling Guterres on Tuesday that he still hoped negotiations with Kyiv would bear fruit.
"We have all the tools for this, that no one else can boast of having," the Russian leader told politicians, 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.
However later Wednesday In Brussels, Ursula von der Leyen, the president of the European Commission, said Poland and Bulgaria are now receiving gas from their EU neighbours.
'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 neighbours," she said.
"The era of Russian fossil fuels in Europe will come to an end."
EU officials said energy ministers from across the bloc will meet on Monday to discuss the situation.
European powers have imposed massive sanctions on Russia since Putin's decision to invade his neighbour, 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.
Vladimir 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 Ms von der Leyen said that "about 97 per cent" 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.
The European Commission on Wednesday sought to lend Kyiv economic support by proposing a suspension of import duties on Ukrainian goods, but the idea still needs to be approved in a vote by the bloc's 27 members.
'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."
A Ukrainian serviceman walks amid the rubble of a building heavily damaged by multiple Russian bombardments near a frontline in Kharkiv, Ukraine, on Monday, 25 April, 2022. Source: AAP, AP / Felipe Dana
The campaign has refocused on seizing the east and south of the country, while increased the use of long-range missile strikes against west and central Ukraine to counter the Western response.
Ukraine's Defence Minister Oleksiy Reznikov predicted "extremely difficult weeks" for the country amid "destruction and painful casualties" during the offensive.
Russia's defence 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 aluminium plant near the Ukrainian city of Zaporizhzhia with "high-precision long-range sea-based Kalibr missiles", the ministry said.
It also accused Ukraine of preparing to stage a fake civilian massacre in Lysychansk by disguising the bodies of dead Ukrainian soldiers in civilian clothing and taking them to the city's central market.
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 unrecognised 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.
Moldova's Foreign Minister Nicu Popescu called the events a "dangerous deterioration of the situation".
Mr Popescu said the Transnistrian authorities announced they would prevent men of fighting age from leaving the region, calling it "a sign we are not yet out of the potential danger zone".
Russia's targeting of Western-supplied arms came as the United States and Europe have started to heed Mr Zelenskyy'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.
Residential buildings of the Saltivka District in Kharkiv, Ukraine, have been heavily damaged by daily Russian shelling and rocket attacks since the initial days of the war. Source: AAP, SIPA USA / Justin Yau
Britain was set Wednesday to 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 Mr Putin.
Fighting continues to rage across Ukraine's east, Kyiv's defence 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.
The UN tourism body added to Russia's isolation on the international scene as most of its 159 members on Wednesday voted to suspend it from the agency.
UN Secretary-General Antonio Guterres announced his arrival in Kyiv on Wednesday following talks in Moscow with Russian President Vladimir Putin.
"I have arrived in Ukraine after visiting Moscow," he wrote on his official Twitter account as he landed ahead of talks with Ukraine's President Volodymyr Zelenskyy.
"We will continue our work to expand humanitarian support and secure the evacuation of civilians from conflict zones. The sooner this war ends, the better -- for the sake of Ukraine, Russia, and the world," Mr Guterres tweeted.
At the Moscow talks on Tuesday, Guterres repeated calls for both Russia and Ukraine to work together to set up "safe and effective" humanitarian corridors in war-torn Ukraine.
In turn, Putin told him he hoped that negotiations could end the conflict which saw Russian troops invading Ukraine on 24 February.
"Despite the fact that the military operation is ongoing, we still hope that we will be able to reach agreements on the diplomatic track," the Russian leader said in televised remarks.
Talks had been taking place in Turkey but stalled after the discovery of civilian bodies in areas near Kyiv previously occupied by Russian forces.
