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Russia has stepped up its assaults on eastern and southern Ukraine and President Vladimir Putin has threatened "lightning-fast" retaliation against any Western countries that intervene on Ukraine's behalf.
More than two months into an invasion that has flattened cities but failed to capture the capital Kyiv, Russia has mounted a push to seize two eastern provinces in a battle the West views as a decisive turning point in the war.
"The enemy is increasing the pace of the offensive operation. The Russian occupiers are exerting intense fire in almost all directions," Ukraine's military command said of the situation on the main front in the east on Thursday.
Although Russian forces were pushed out of northern Ukraine last month, they are heavily entrenched in the east and also still hold a swathe of the south that they seized in March.
Ukraine said there were strong explosions overnight in the southern city of Kherson, the only regional capital Russia has captured since the invasion. Russian troops there used tear gas and stun grenades on Wednesday to suppress pro-Ukrainian demonstrations, and were now shelling the entire surrounding region and attacking towards Mykolaiv and Kryvyi Rih, President Vladimir Zelenskiy's southern home city, Ukraine said.
Kyiv accuses Moscow of planning to stage a fake independence referendum in the occupied south. Russian state media quoted an official from a self-styled pro-Russian "military-civilian commission" in Kherson as on Thursday saying the area would start using Russia's rouble currency from May 1.
Western countries have ramped up weapons deliveries to Ukraine in recent days as the fighting in the east has intensified. More than 40 countries met this week at a US air base in Germany and pledged to send heavy arms such as artillery for what is expected to be a vast battle of opposing armies along a heavily fortified front line.
Washington says it hopes Ukrainian forces can not only repel Russia's assault on the east, but weaken its military so that it can no longer threaten neighbours. Russia says that amounts to NATO waging "proxy war" against it.
Thousands of people have been killed and more than five million refugees have fled abroad since Russia launched its "special military operation" in Ukraine on February 24. Moscow says its aim is to disarm its neighbour and defeat nationalists there. The West calls that a bogus pretext for a war of aggression.
US President Joe Biden is expected to deliver remarks on Thursday in support of Ukrainians, the White House said.
While Russia presses its military assault in eastern and southern Ukraine, its economic battle with the West threatens gas supplies to Europe and is battering the Russian economy.
On Wednesday, Moscow halted gas deliveries to Poland and Bulgaria for refusing to pay for supplies in roubles, its first big retaliatory strike against sanctions. The president of the European Commission called the move "blackmail".
"The sooner everyone in Europe recognises that they cannot depend on Russia for trade, the sooner it will be possible to guarantee stability in European markets," Zelenskiy said in an overnight address.
Germany, the biggest buyer of Russian energy, hopes to stop importing Russian oil within days, but untangling Europe from Russian gas would be far more disruptive. Berlin said a Russian energy embargo or blockade would tip Europe's largest economy into recession.
Concern has also increased over the prospect of the conflict widening to neighbouring Moldova, where pro-Russian separatists have blamed Ukraine for reported attacks this week in their region, occupied since the 1990s by Russian troops. Kyiv and Moldova's pro-Western government call the incidents there provocations.