Zelenskyy said that Russia wants to occupy other countries and that the invasion of Ukraine may be the beginning of a campaign of Russian expansion.
Zelenskyy said in a video address on Friday night that the Russian invasion of Ukraine was only the beginning.
Zelenskyy asked nations not to remain neutral in the conflict because they could be in danger.
He said that the nations that believe in the victory of life over death must fight with them. Who is next?
The comments followed reports that one of the Kremlin's leading military figures raised the prospect of Russian expansion into Ukraine's neighbor.
The deputy commander of Russia's Central Military District was quoted in state media as saying that Russia wants total control over the south of Ukraine.
Reports say that it would give Russia access to a Russian-backed region of Moldova.
Control over the south of Ukraine is another way out to Transnistria, where there are facts of oppression of the Russian-speaking population, according to the Russian media.
The western border of Ukraine is next to Transnistria. It is a state that is recognized by Russia as part of Moldova. Russian troops have been based there since 1992.
The foreign ministry described the comments as "deeply concerning", according to the report.
The claim that the Russian-speaking population of Transnistria is facing oppression is false, according to the government.
Russian President Putin made similar claims about the oppression of Russian speakers in Ukraine before the invasion.