"They are also planning to involve more foreigners," Zelenskiy said in his evening video address, citing intelligence from Ukrainian security services.
There has been speculation for months about a new mobilisation in Russia to offset the recent heavy losses on the front lines in Ukraine.
Russian President Vladimir Putin has so far categorically rejected such fears.
Zelenskiy said there was also new information regarding the presence of North Korean soldiers in Russia.
"There are currently more than 8000 North Korean soldiers on Russian territory," Zelenskiy said.
"In addition, preparations are under way to deploy a contingent of a further 10,000 soldiers - these are being selected to undergo training and will subsequently be transferred to Russia."
Russia is paying for this with technology, Zelenskiy stated.
He said in return for the assistance, production of Shahed-type attack drones had commenced on North Korean territory with technical support from Russia.
"This is how evil spreads, and this is how a threat to life in one part of the world spreads to other parts of the world," Zelenskiy asserted.
His comments came after a drone struck Ukrainian territory 2km from a border crossing with Poland on Monday, damaging a border checkpoint but causing no casualties, according to Polish and Ukrainian officials.
"A jet-powered drone struck on the Ukrainian side, about two kilometres from the border crossing with Poland at Dorohusk," Polish Defence Minister Wladyslaw Kosiniak-Kamysz wrote on X.
Polish airspace was not violated, he added.
The blast wave from the explosion damaged the border checkpoint building on the Ukrainian side, according to Roman Romaniuk, head of the administration in Ukraine's Volyn region.
"There have been no reports of injuries or deaths," Romaniuk wrote on Telegram.
The Polish armed forces command said an air-raid alert was triggered in the Lublin region of southeastern Poland during Russian air strikes against western Ukraine.
The alert was lifted after about 15 minutes.
There have been several Russian drone attacks on Ukrainian territory near the Dorohusk-Yahodyn border crossing over the past two weeks.
A passenger train travelling from Kyiv to Warsaw was also attacked in the area.
The Polish government has accused Russia of escalating its war against Ukraine.
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