Zelenskiy told Luxembourg's parliament via videolink on Thursday that Russian forces now occupied about a fifth Ukrainian territory, with battle lines now stretching more than 1,000 km.
As the invasion heads for its 100th day on Friday, Russia says Washington is adding "fuel to the fire" with a new $US700 million ($A965 million) weapons package for Ukraine that will include advanced rocket systems with a range of up to 80 km.
But separately addressing a forum in Slovakia, Zelenskiy called for more weapons supplies to "ensure an inflection point in this confrontation," in Ukraine's favour.
US President Joe Biden's administration said it had Ukraine's assurances it would not use the rocket systems to hit targets inside Russia.
"Ukraine is fighting an exclusively defensive war, and we always state this," the country's deputy defence minister Hanna Malyar told a briefing, when asked whether Kyiv made such a promise.
While Moscow denies targeting civilians it says it regards Ukrainian infrastructure transport used to bring in Western arms as a legitimate target. But it downplayed the effect those supplies will have on what it calls its "special military operation" to disarm Ukraine and rid it of ultra-nationalists the Kremlin says threaten Russian security.
"Pumping (Western) weapons into Ukraine does not change all the parameters of the special operation," Kremlin spokesman Dmitry Peskov told reporters.
"Its goals will be achieved, but this will bring more suffering to Ukraine," Peskov said in response to a question whether U.S. plans to sell Ukraine drones that can be armed with missiles could change the nature of the conflict.
Russian forces, backed by heavy artillery, control most of the eastern industrial city of Sievierodonetsk - now largely in ruins - after days of fierce fighting in which they have taken losses, Britain's defence ministry said in its daily intelligence report.
Ukraine's armed forces general staff said that besides its assault on the city, Russian troops were also attacking other parts of the east and northeast.
The capture of Sievierodonetsk and its smaller twin Lysychansk would give Russian forces control of all of Luhansk, one of two provinces along with Donetsk in the Donbas claimed by Moscow on behalf of separatists.
Seizing Luhansk would fulfil one of Russian President Vladimir Putin's stated aims and further shift battlefield momentum in Russia's favour after its forces were pushed back from the capital Kyiv and from northern Ukraine.
Moscow's forces were also attempting to advance south towards the Ukraine-held cities of Kramatorsk and Sloviansk, in Donetsk province, provincial governor Pavlo Kyrylenko said.
The war and Western sanctions imposed in response to the February 24 invasion are having a massive impact on the world economy. With its control of some of Ukraine's biggest seaports and critical Black Sea shipping routes, Russia has been blocking Ukrainian farm exports and deepening a global food crisis.
Russia and Ukraine together account for nearly a third of global wheat supplies, while Russia is also a key fertiliser exporter and Ukraine a major supplier of corn and sunflower oil.
Signalling a possible breakthrough, Interfax news agency quoted Russia's defence ministry as saying on Thursday that vessels carrying grain will be allowed to leave Ukraine's Black Sea ports via "humanitarian corridors" with Moscow ready to guarantee their safety.
