"What they are doing right now ... could put a stop to any form of negotiation," Zelensky said in an interview with a Ukrainian news website published on Saturday.
"There are troops there who absolutely hate them, and I don't think they will let them live," the Ukrainian leader said, referring to the fact that many of the fighters trapped in the city are part of the nationalist-dominated Azov Batallion, whose far-right leanings have been the subject of Russian media reports for weeks.
The remaining Ukrainian fighters are holed up in the Azovstal steelworks, and Zelensky confirmed that the Ukrainian authorities remained in contact with them.
Following the Russian invasion of Ukraine on February 24, Mariupol was completely surrounded by Russian troops in early March, since when it has been under siege.
Zelensky once again rejected Russian demands that Ukraine cede its territory to Moscow. "We do not trade territories and our people," he said, adding that negotiations between the two sides were nevertheless necessary to end the fighting and to defend the Ukrainian state.
Earlier on Saturday, the Russian defence ministry announced it had cleared the entire urban area of Mariupol of Ukrainian forces - other than the few fighters who remained in the Azovstal steelworks.
In an online post on Saturday, the ministry said that as of April 16, Ukrainian forces in the besieged port city had lost more than 4000 people, RIA added.
Russian forces have been trying for several weeks to take the port, which is on the Sea of Azov, a body of water to the northeast of the Black Sea.
"The entire urban area of Mariupol has been completely cleared ... remnants of the Ukrainian group are currently completely blockaded on the territory of the Azovstal metallurgical plant," the ministry said.
"Their only chance to save their lives is to voluntarily lay down their arms and surrender."
There was no immediate reaction from Kyiv to the statement by the Russian ministry, which also said 1464 Ukrainian servicemen had surrendered so far.
Moscow said the total number of what it called "irretrievable losses" suffered by Ukraine totaled 23,367 people but did not provide any evidence and did not say whether this included only those who had died or who had also been injured.
AAP with The Project