Poroshenko paid tribute to Nemtsov, who was shot dead late on Friday, and said the fierce critic of President Vladimir Putin had told him a couple of weeks ago that he had proof of Russia's role in the Ukraine crisis and would reveal it.
"He said he would reveal persuasive evidence of the involvement of Russian armed forces in Ukraine. Someone was very afraid of this ... They killed him," Poroshenko said in televised comments during a visit to the city of Vinnytsia.
Nearly 6,000 people have been killed since pro-Russian separatists rebelled in east Ukraine last April, after the ousting of a Moscow-backed president in Kiev and Russia's annexation of the Crimea peninsula.
Kiev and its Western allies say the rebels are funded and armed by Moscow, and backed by Russian military units.
Moscow denies aiding sympathizers in Ukraine, and says heavily armed Russian-speaking troops operating without insignia there are not its men.
Source: The World Post