Mykhaylo Podolyak told the BBC that Ukraine needs more weapons to match Russia’s firepower.
A senior Ukrainian presidential aide has told the BBC that between 100 and 200 Ukrainian troops are being killed on the front line every day.
Mykhaylo Podolyak said Ukraine needed hundreds of Western artillery systems to level the playing field with Russia in the eastern Donbas region.
He also said Kyiv is not ready to resume peace talks with Moscow.
Ukrainian troops are under relentless bombardment as Russian forces attempt to take control of the whole of Donbas.
“The Russian forces have thrown pretty much everything non-nuclear at the front and that includes heavy artillery, multiple rocket launch systems and aviation,” Mr Podolyak said.
He repeated Ukraine’s appeal for more weapons from the West, saying that the “complete lack of parity” between the Russian and Ukrainian armies was the reason for Ukraine’s heavy casualty rate.
“Our demands for artillery are not just some kind of whim… but an objective need when it comes to the situation on the battlefield,” he said, adding that Ukraine needs 150 to 300 rocket launch systems to match Russia – a much higher number than it has received so far.
Mr Podolyak also said peace talks could only resume if Russia surrendered the territory it had gained since it invaded on 24 February.
Mr Podolyak’s suggestion that 100 to 200 Ukrainian soldiers are dying each day is higher than previous estimates. On Thursday, Ukraine’s Defence Minister, Oleksii Reznikov, said Ukraine was losing 100 soldiers a day, and 500 more were injured.
The differing casualty figures are a sign of how difficult it is to get precise information from the battlefield.
Despite the heavy losses, Mr Reznikov claimed that a high number of Russian soldiers are also being killed.
“The Kremlin continues to press by sheer mass, stumbles, faces strong rebuffs and suffers huge casualties,” Mr Reznikov said. “But yet still has forces to advance in some parts of the front.”
Similarly, Luhansk regional governor Sergei Gaidai said Russians were “dying like flies” but echoed Mr Podolyak’s claim that Ukraine is facing difficulties with a “catastrophic” lack of artillery.
Russian forces have concentrated their assault on the city of Severodonetsk. On Wednesday Mr Zelensky said “the fate of the Donbas is being decided there” and officials said it has been reduced to rubble by intense Russian artillery and missile barrages.
In addition to the frontline fighting, two Britons and a Moroccan man who fought for Ukraine’s armed forces were sentenced to death on Thursday by an unrecognised court in the so-called Donetsk People’s Republic.
They were found guilty of being mercenaries and of “taking action towards the violent seizure of power”.