Zelenskiy said Ukrainian intelligence indicated Belarus had begun expanding roads and building artillery positions in areas near Ukraine's border.
"We believe that Russia will once again try to involve Belarus in its war," Zelenskiy said on social media.
He did not provide evidence for the construction activities in Belarus.
In February 2022, when Russia launched its full-scale invasion of Ukraine, it also entered Ukrainian territory from Belarus.
Zelenskiy said on Friday that Ukraine had issued instructions to warn the Belarusian leadership of "Ukraine's readiness to defend its land and independence".
He also said Belarus's government should take note of what happened to Venezuela's leader.
"The nature and the consequences of the most recent events in Venezuela should prevent Belarus's leadership from making mistakes," Zelenskiy said.
In January the US military captured Venezuela's head of state Nicolas Maduro and took him and his wife to New York where they have been jailed ever since, facing charges of drug trafficking conspiracies, among other things.
Meanwhile, a fire broke out at an oil terminal in southern Russia's Krasnodar region, prompting a large firefighting operation, the region's emergency operational headquarters said.
No casualties were reported and there was no indication as to what may have caused the fire.
The emergency headquarters, writing on Telegram early on Saturday, local time, said 224 firefighters and 56 vehicles were battling the blaze at the terminal in Tikhoretsk, northeast of the region's main town, Krasnodar.
Crews from Russia's emergencies ministry were also taking part.
Ukraine's top commander Oleksandr Syskyi said this week that Ukrainian forces were seeking to reduce Russia's offensive capabilities by keeping up a high pace of strikes on military, defence-industrial and other facilities.
In March, Ukraine struck 76 such targets including 15 facilities in the oil-refining industry, he said.
with DPA
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