“Today, about 20 percent of our territory is under the control of the occupiers,” he said during an address to lawmakers in Luxembourg on Thursday.
“This is what it means, in fact, to characterise this war as full-scale. This is why we are calling the world for their support.”
His comments came as Russian troops consolidated their control over the industrial city of Sievierodonetsk after days of fierce fighting. Kyiv said that Ukrainian troops had taken the strategic decision to withdraw from the city, which lies in the Luhansk region.
Serhiy Haidai, governor of the eastern Luhansk region, said that Ukrainian forces had retreated to “more advantageous, pre-prepared positions” while awaiting western weapons.
Mr Zelensky warned that if Russia prevailed in its war then “dark times will come for everyone” in Europe.
"If we win this war, all Europeans will be able to continue enjoying their freedom,” he told parliamentarians in Luxembourg.
“But if this one person who wants to destroy any freedom in Ukraine and Europe prevails, then dark times will come for everyone on the continent,” he added, referring to the Russian president, Vladimir Putin.
Kyiv is set to receive further deliveries of western weapons in the coming days as Mr Zelensky’s forces seek to stem the Russian advance in the Donbas.
Britain said Thursday that it is sending sophisticated medium-range rocket systems to Ukraine. The pledge came a day after the United States and Germany said they would equip the embattled nation with advanced weapons for shooting down aircraft and knocking out artillery.
It follows repeated calls from Ukraine for its western allies to send longer-range missiles to help it counter Russian artillery strikes in the Donbas.
Kremlin spokesperson Dmitry Peskov warned of “absolutely undesirable and rather unpleasant scenarios” if Ukrainian soldiers “hypothetically try to use these weapons against targets on our territory”.
In other developments, the US issued a fresh round of sanctions on Thursday targeting 17 individuals, including Sergei Roldugin, a close associate of Mr Putin.
Maria Zakharova, the spokeswoman of the Russian Ministry of Foreign Affairs, is also on the list.
“Russia’s elites, up to and including President Putin, rely on complex support networks to hide, move, and maintain their wealth and luxury assets,” said Brian Nelson, undersecretary of the Treasury for Terrorism and Financial Intelligence.
“Today’s action demonstrates that Treasury can and will go after those responsible for shielding and maintaining these ill-gotten interests. We will continue to enforce our sanctions and expose the corrupt systems by which President Putin and his elites enrich themselves.”