With Russian forces retreating from front lines in the south and east, Mr Zelensky said in a late-night video address on Wednesday that Novovoskresenske, Novohryhorivka and Petropavlivka to the northeast of Kherson city had been “liberated”.
Addressing pro-Moscow forces in Russian, he said: “You have already lost.
“Ukrainians know what they are fighting for. And more and more citizens of Russia are realising that they must die simply because one person does not want to end the war. It’s obvious who will win.”
His comments come after a senior US source told The Telegraph on Wednesday there “is now a distinct possibility” of Ukraine reclaiming Crimea, which was annexed by Russia in 2014.
“It is clear that Russia no longer has the ability or willpower to defend key positions, and if the Ukrainians succeed in their goal of recapturing Kherson, then there is a very real possibility that it will ultimately be able to recapture Crimea,” said the source.
The comments were echoed by US deputy assistant secretary of defence Laura Cooper.
On Wednesday, Russian president Vladimir Putin appeared to make a rare concession that his attempts to absorb the Donetsk, Luhansk, Kherson and Zaporizhzhia have hit issues, saying: “We are working on the assumption that the situation in the new territories will stabilise.”
At the United Nations, Russia is lobbying for a secret ballot instead of a public vote next week when the 193-member UN General Assembly considers whether to condemn its annexation of the four regions, after staging ‘referendums’ in the provinces.
Mr Putin signed a law on Wednesday to incorporate the regions into Russia.
The new law would incorporate about 18 per cent of Ukraine’s territory into Russia - equivalent to the area of Portugal - in Europe’s biggest annexation since the Second World War.
Mr Putin says he wants to ensure Russia’s security and protect Russian-speakers in Ukraine, but Kyiv accuses Moscow of a land grab for territory.
Ukraine says it will never accept an illegal seizure of its territory by force, while Kyiv and the West say the referendums were rigged votes held at gunpoint.
Russia’s move to annex the regions raises the possibility of escalation in the war. Putin and other officials have said they could use nuclear weapons to protect Russian territory including the annexed provinces.
Ukraine has said it will not be swayed by any nuclear threats and Mr Zelensky said in his address he and his senior military officials met to discuss recovering all lands occupied by Russia.