Acknowledging that Kyiv's stance on the war was becoming more uncompromising, presidential adviser Mykhailo Podolyak said making concessions would backfire on Ukraine because Russia would hit back harder after any break in fighting.
"The war will not stop (after any concessions). It will just be put on pause for some time," he told Reuters in an interview in the heavily guarded presidential office, where some of the windows and corridors are protected by sandbags.
"After a while, with renewed intensity, the Russians will build up their weapons, manpower and work on their mistakes, modernise a little, fire many generals ... And they'll start a new offensive, even more bloody and large-scale."
Podolyak dismissed as "very strange" calls in the West for an urgent ceasefire that would involve Russian forces remaining in territory they have occupied in Ukraine's south and east.
"The (Russian) forces must leave the country and after that the resumption of the peace process will be possible," he said.
Both sides say peace talks have stagnated. Each blames the other.
Thousands of people have been killed, millions have been displaced and towns and cities have been devastated since Russia invaded on Feb. 24. [nL2N2XD01M
Russia says it has taken full control of the southern city of Mariupol in what it calls its "special military operation" in Ukraine, but its invasion has stalled in other areas and Ukraine has been bolstered by increasing arms supplies from its allies.
A ceasefire would play into the Kremlin's hands, Podolyak said.
"They want to lock in some kind of military successes. There will definitely be no military successes given the help from our Western partners," he said.
"It would be good if the European and U.S. elites understand to the end: Russia can't be left halfway because they will (develop) a 'revanchist' mood and be even more cruel ... They must be defeated, be subjected to a painful defeat, as painful as possible."
(Editing by Timothy Heritage)
By Tom Balmforth