Brussels spearheaded a powerful show of European solidarity on Friday by backing Kyiv's bid for EU candidate status, an endorsement that could add Ukraine to the list of countries vying for membership as early as next week.
The European Commission is set to meet at their Brussels summit on Thursday with all 27 leaders already backing Kyiv's candidacy and the heads of the bloc's biggest members -- France, Germany and Italy -- giving full-throated support to the idea.
Even though EU membership could still be years away, Zelensky called the decision a "historic achievement".
"Ukrainian institutions maintain resilience even in conditions of war. Ukrainian democratic habits have not lost their power even now," Zelensky said in a video address.
His comments came as powerful shelling struck the country's Donbas region - large parts of which have been occupied by Russian troops - sparking a humanitarian catastrophe that "continues to deteriorate rapidly," according to the UNOCHA.
Hundreds of people are trapped in makeshift bomb shelters in the key city of Severodonetsk, which is in the Lugansk region, including at the besieged Azot chemical plant, governor Sergiy Gaiday said.
"It is now impossible and physically dangerous to get out of the plant due to constant shelling and fighting. There are 568 people in the shelter, including 38 children," he said.
Gaiday said this week that around 10,000 civilians remained in the city, which is controlled mostly by Russian forces.
The Ukrainian army said on Saturday there was continuous "fire from artillery and rocket-propelled grenade launchers at the positions of our troops and civilian infrastructure".
Russian forces also launched missile strikes on Ukraine's largest oil refinery on Saturday, according to the regional governor in the central Poltava region.
"Kremenchuk (oil refinery) is again under enemy strike. Between 6 and 8 Russian missiles hit refineries and other infrastructure," Dmytro Lunin said in a Telegram message.
In the city of Lysychansk, also in Donbas and the target of Russian shelling, residents were preparing to be evacuated.
"We're abandoning everything and going. No one can survive such a strike," said history teacher Alla Bor, waiting with her son-in-law Volodymyr and 14-year-old grandson.
"We are abandoning everything, we are leaving our house. We left our dog with food. It's inhumane but what can you do?"
Russian state television also aired social media videos of two US military veterans who went missing last week while fighting alongside the Ukrainian army, stating they had been captured by Russian forces.
On Friday, United States President Joe Biden had said he did not know the whereabouts of lexander Drueke and Andy Huynh, after their relatives lost contact with the pair.
The missing Americans -- including a third identified as a former US Marines captain -- are believed to be part of an unknown number of mostly military veterans who have joined other foreigners to volunteer alongside Ukrainian troops.
Russian President Vladimir Putin said he had "nothing against" Ukraine joining the EU, saying it was "their sovereign decision to join economic unions or not" -- unlike the security risk he sees in Kyiv joining NATO.
But he said European Union membership would turn Ukraine into a "semi-colony" of the West.
On Friday, the European Commission gave formal backing to the bid, and EU chief Ursula von der Leyen made her position clear by donning a striking jacket in Ukraine's national colours.
"We all know that Ukrainians are ready to die for the European perspective. We want them to live with us for the European dream," she said.
Once Ukraine joins the EU candidates' list -- alongside several countries in the Western Balkans -- it could still take years to meet all the formal membership requirements, even if Kyiv prevails in the war.
Moscow has turned up the pressure on Western allies by sharply reducing flows of natural gas in its pipelines to western Europe, driving up energy prices.
France's network provider said it had not received any Russian gas by pipeline from Germany since June 15, and Italy's Eni said it expected Russian firm Gazprom to cut its supplies by half on Friday.
Several European countries, including Italy and Germany, are highly reliant upon Russian gas for their energy needs.
Berlin and Rome have rejected Russia's argument that technical issues have caused the drop in supplies, arguing that state-owned Gazprom's move is political.
But western Europe is sweltering in a heatwave and energy prices are already soaring, adding to runaway inflation and triggering industrial action in several economies.
Separately, Zelensky announced an end to the visa-free travel that Russian citizens, many of whom have Ukrainian relatives, have enjoyed since Ukraine became independent from the Soviet Union in 1991.