Zelensky has said that if elected, he would demand Russia end its occupation of territory in eastern Ukraine and pay compensation -- a prospect widely seen as unrealistic.

Russian President Vladimir Putin's spokesman Dmitry Peskov told journalists Moscow is "closely following" candidates' statements on the conflict.

Kiev and Western powers accuse Russia of giving military support to separatists in the east.

"Of course we would like to see not a party of war in control in Ukraine but a party that is aimed at real, step-by-step resolution of the situation in southeastern Ukraine," Peskov said.

Asked to comment on Zelensky's stance on the Russian occupation, Peskov said Crimea, annexed by Russia in 2014, "is not up for discussion."

He said the conflict "is a consequence of the policies carried out by Ukraine's leadership".

Ukrainian President Petro Poroshenko was elected in 2014 on a promise to end the fighting in eastern Ukraine.

Under Poroshenko, Kiev has sought closer ties with the West as the war rumbles on, with a death toll now estimated at around 13,000.

Putin's spokesman said it would be "premature" to comment on the significant lead taken by comedian Zelensky in the first round of voting on Sunday. It will be followed by a run-off vote on April 21 between Zelensky and Poroshenko.

Partial results published on Monday morning showed Zelensky taking 30 percent in Sunday's first round, well ahead of Poroshenko on roughly 17 percent.